ci: add coreutils to linux image

The `run-task` script now uses `env -S` which is not a supported flag in
the busybox implementation of `env`. Install coreutils to use the
standard version.

taskcluster/docker/linux/Dockerfile

@@ -15,7 +15,7 @@ RUN apk add --no-cache python3 py3-pip && \
     ln -sf python3 /usr/bin/python
 
 # Setup other dependencies
-RUN apk add bash git
+RUN apk add bash coreutils git
 
 # %include-run-task