CI: Cache tree-sitter grammars in all jobs

This change adds tree-sitter grammar caching to Check, Lints and Docs
jobs which all previously downloaded grammars in the `helix-term` build
script fresh per job. This should increase reliability and mitigate the
effects of an ongoing SourceHut outage
(<https://status.sr.ht/issues/2025-01-23-git.sr.ht-ddos/>).

This is also a nice speed boost for these jobs:

| Job name | Example time before | Example time after |
|---       |---                  |---                 |
| Check    | 2m20s               | 47s                |
| Lints    | 2m56s               | 1m10s              |
| Docs     | 4m56s               | 2m35s              |

.github/workflows/build.yml

@@ -10,6 +10,8 @@ on:
 
 env:
   MSRV: "1.76"
+  # This key can be changed to bust the cache of tree-sitter grammars.
+  GRAMMAR_CACHE_VERSION: ""
 
 jobs:
   check:
@@ -29,6 +31,13 @@ jobs:
         with:
           shared-key: "build"
 
+      - name: Cache tree-sitter grammars
+        uses: actions/cache@v4
+        with:
+          path: runtime/grammars
+          key: ${{ runner.os }}-stable-v${{ env.GRAMMAR_CACHE_VERSION }}-tree-sitter-grammars-${{ hashFiles('languages.toml') }}
+          restore-keys: ${{ runner.os }}-stable-v${{ env.GRAMMAR_CACHE_VERSION }}-tree-sitter-grammars-
+
       - name: Run cargo check
         run: cargo check
 
@@ -52,12 +61,12 @@ jobs:
         with:
           shared-key: "build"
 
-      - name: Cache test tree-sitter grammar
+      - name: Cache tree-sitter grammars
         uses: actions/cache@v4
         with:
           path: runtime/grammars
-          key: ${{ runner.os }}-stable-v${{ env.CACHE_VERSION }}-tree-sitter-grammars-${{ hashFiles('languages.toml') }}
-          restore-keys: ${{ runner.os }}-stable-v${{ env.CACHE_VERSION }}-tree-sitter-grammars-
+          key: ${{ runner.os }}-stable-v${{ env.GRAMMAR_CACHE_VERSION }}-tree-sitter-grammars-${{ hashFiles('languages.toml') }}
+          restore-keys: ${{ runner.os }}-stable-v${{ env.GRAMMAR_CACHE_VERSION }}-tree-sitter-grammars-
 
       - name: Run cargo test
         run: cargo test --workspace
@@ -87,6 +96,13 @@ jobs:
         with:
           shared-key: "build"
 
+      - name: Cache tree-sitter grammars
+        uses: actions/cache@v4
+        with:
+          path: runtime/grammars
+          key: ${{ runner.os }}-stable-v${{ env.GRAMMAR_CACHE_VERSION }}-tree-sitter-grammars-${{ hashFiles('languages.toml') }}
+          restore-keys: ${{ runner.os }}-stable-v${{ env.GRAMMAR_CACHE_VERSION }}-tree-sitter-grammars-
+
       - name: Run cargo fmt
         run: cargo fmt --all --check
 
@@ -115,6 +131,13 @@ jobs:
         with:
           shared-key: "build"
 
+      - name: Cache tree-sitter grammars
+        uses: actions/cache@v4
+        with:
+          path: runtime/grammars
+          key: ${{ runner.os }}-stable-v${{ env.GRAMMAR_CACHE_VERSION }}-tree-sitter-grammars-${{ hashFiles('languages.toml') }}
+          restore-keys: ${{ runner.os }}-stable-v${{ env.GRAMMAR_CACHE_VERSION }}-tree-sitter-grammars-
+
       - name: Validate queries
         run: cargo xtask query-check