Restores the tar command to its original state (#21)

### What
This PR tries to reduce side effects by:
- Restores the `tar` command to its original state
- Narrow the ~/.cache directory to just nix and devbox

### Why
We are using `chmod u+s "$(command -v tar)"` to setuid on tar (This is
needed because we are caching `/nix/` and other directories outside of
`~`, which is owned by `root`)

This has a problem where it changes the parent directory of
`~/.cache/nix` to be owned by `root` instead of `runner`.

I thought #22
had fixed it, but we ran into similar issues when we use a go cache on
top of the devbox cache.

### Caveat
Cache enabled on self hosted Linux may stop working

Author: LucilleH

action.yml

@@ -100,8 +100,12 @@ runs:
           echo "#!/bin/sh" >> /usr/local/bin/gtar
           echo 'exec sudo /usr/local/bin/gtar.orig "$@"' >> /usr/local/bin/gtar
           sudo chmod +x /usr/local/bin/gtar
-        else
-          sudo chmod u+s "$(command -v tar)"
+        elif [ "$RUNNER_OS" == "Linux" ]; then
+          mkdir -p ~/.cache
+          mkdir -p ~/.local/bin
+          echo "#!/bin/sh" >> ~/.local/bin/tar
+          echo 'exec sudo /usr/bin/tar "$@"' >> ~/.local/bin/tar
+          sudo chmod +x ~/.local/bin/tar
         fi
 
     - name: Configure nix access-tokens
@@ -122,7 +126,8 @@ runs:
       uses: actions/cache@v3
       with:
         path: |
-          ~/.cache
+          ~/.cache/devbox
+          ~/.cache/nix
           ~/.local/state/nix
           ~/.nix-defexpr
           ~/.nix-profile
@@ -134,3 +139,13 @@ runs:
       shell: bash
       run: |
         devbox run --config=${{ inputs.project-path }} -- echo "Packages installed!"
+
+    - name: Restore tar command
+      if: inputs.enable-cache == 'true'
+      shell: bash
+      run: |
+        if [ "$RUNNER_OS" == "macOS" ]; then
+          sudo mv /usr/local/bin/gtar.orig /usr/local/bin/gtar
+        elif [ "$RUNNER_OS" == "Linux" ]; then
+          rm ~/.local/bin/tar
+        fi