Ensure that the pip cache directory is writable by the astro user (#19)

Otherwise it's owned by root and astro can't actually cache anything as we are installing the virtualenvs

Build output show it's being used:

```
#25 [stage-0 14/17] RUN --mount=type=cache,uid=50000,gid=0,target=/home/astro/.cache/pip /home/astro/.venv/snowpark/bin/pip --cache-dir=/home/astro/.cache/pip install -r /home/astro/.venv/
snowpark/requirements.txt
#25 0.902 + /home/astro/.venv/snowpark/bin/pip --cache-dir=/home/astro/.cache/pip install -r /home/astro/.venv/snowpark/requirements.txt
#25 2.110 Looking in indexes: https://pip.astronomer.io/v2/
#25 2.110 Looking in links: https://pip.astronomer.io/simple/astronomer-fab-security-manager/, https://pip.astronomer.io/simple/astronomer-airflow-version-check/
#25 2.645 Collecting googleads
#25 2.649   Using cached googleads-38.0.0.tar.gz (27 kB)
#25 2.664   Preparing metadata (setup.py): started
#25 3.407   Preparing metadata (setup.py): finished with status 'done'
```


And in the built image

```
❯ docker run --rm -ti 90a591231bee0e0312a995fd3ed017600f86a054ec1176bc84bef85e1c245aac bash -c 'ls -al /home/astro/.cache/pip'
total 8
drwxr-xr-x 2 astro astro 4096 Jun 16 21:54 .
drwxr-xr-x 3 astro astro 4096 Jun 16 21:54 ..
```

buildkit/internal/transform/transforms.go

@@ -26,18 +26,20 @@ RUN /sbin/ldconfig /usr/local/lib
 RUN ln -s /usr/local/include/python{{.PythonMajorMinor}} /usr/local/include/python{{.PythonMajorMinor}}m
 USER astro
 `
-	virtualEnvTemplate = `RUN mkdir -p /home/astro/.venv/{{.Name}}
-{{if .RequirementsFile}}COPY {{.RequirementsFile}} /home/astro/.venv/{{.Name}}/requirements.txt{{end}}
+	virtualEnvTemplate = `RUN mkdir -p /home/astro/.cache/pip /home/astro/.venv/{{.Name}}
+{{if .RequirementsFile}}COPY --chown={{.AstroUid}}:0 {{.RequirementsFile}} /home/astro/.venv/{{.Name}}/requirements.txt{{end}}
 RUN /usr/local/bin/python{{.PythonMajorMinor}} -m venv /home/astro/.venv/{{.Name}}
 ENV ASTRO_PYENV_{{.Name}} /home/astro/.venv/{{.Name}}/bin/python
-{{if .RequirementsFile}}RUN --mount=type=cache,target=/home/astro/.cache/pip /home/astro/.venv/{{.Name}}/bin/pip --cache-dir=/home/astro/.cache/pip install -r /home/astro/.venv/{{.Name}}/requirements.txt{{end}}
+{{if .RequirementsFile}}RUN --mount=type=cache,uid={{.AstroUid}},gid=0,target=/home/astro/.cache/pip /home/astro/.venv/{{.Name}}/bin/pip --cache-dir=/home/astro/.cache/pip install -r /home/astro/.venv/{{.Name}}/requirements.txt{{end}}
 `
 	fromCommand       = "FROM"
 	argCommand        = "ARG"
 	pyenvCommand      = "PYENV"
 	astroRuntimeImage = "quay.io/astronomer/astro-runtime"
 
 	defaultImageFlavour = "slim-bullseye"
+	// Sadly for the `RUN --mount,uid=$uid` we need to use a numeric ID.
+	defaultAstroUid = 50000
 )
 
 var (
@@ -229,9 +231,18 @@ func (r *Transformer) addVirtualEnvironment(venv *virtualEnv) (*parser.Node, err
 		return nil, err
 	}
 	buf := &bytes.Buffer{}
-	if err := tpl.Execute(buf, venv); err != nil {
+	params := struct {
+		*virtualEnv
+		AstroUid int
+	}{
+		venv,
+		defaultAstroUid,
+	}
+
+	if err := tpl.Execute(buf, params); err != nil {
 		return nil, err
 	}
+
 	parsedNodes, err := parser.Parse(buf)
 	if err != nil {
 		return nil, err

buildkit/internal/transform/transforms_test.go

@@ -31,11 +31,11 @@ RUN /sbin/ldconfig /usr/local/lib
 
 RUN ln -s /usr/local/include/python3.8 /usr/local/include/python3.8m
 USER astro
-RUN mkdir -p /home/astro/.venv/venv1
-COPY reqs/venv1.txt /home/astro/.venv/venv1/requirements.txt
+RUN mkdir -p /home/astro/.cache/pip /home/astro/.venv/venv1
+COPY --chown=50000:0 reqs/venv1.txt /home/astro/.venv/venv1/requirements.txt
 RUN /usr/local/bin/python3.8 -m venv /home/astro/.venv/venv1
 ENV ASTRO_PYENV_venv1 /home/astro/.venv/venv1/bin/python
-RUN --mount=type=cache,target=/home/astro/.cache/pip /home/astro/.venv/venv1/bin/pip --cache-dir=/home/astro/.cache/pip install -r /home/astro/.venv/venv1/requirements.txt
+RUN --mount=type=cache,uid=50000,gid=0,target=/home/astro/.cache/pip /home/astro/.venv/venv1/bin/pip --cache-dir=/home/astro/.cache/pip install -r /home/astro/.venv/venv1/requirements.txt
 COPY foo bar
 USER root
 COPY --link --from=python:3.10-slim-bullseye /usr/local/bin/*3.10* /usr/local/bin/
@@ -47,7 +47,7 @@ RUN /sbin/ldconfig /usr/local/lib
 
 RUN ln -s /usr/local/include/python3.10 /usr/local/include/python3.10m
 USER astro
-RUN mkdir -p /home/astro/.venv/venv2
+RUN mkdir -p /home/astro/.cache/pip /home/astro/.venv/venv2
 
 RUN /usr/local/bin/python3.10 -m venv /home/astro/.venv/venv2
 ENV ASTRO_PYENV_venv2 /home/astro/.venv/venv2/bin/python