GitHub Actions: fix ENOENT for jobs running in forks

https://github.com/peterzhu2118/ruby/actions/runs/5649652235/job/15304434412

> Errno::ENOENT: No such file or directory @ rb_sysopen -
> ./.downloaded-cache/config.guess:
> https://raw.githubusercontent.com/gcc-mirror/gcc/master/config.guess

Unsure about the root cause but making sure the directory is there seems like a
plausible fix. Maybe the issue is sensitive to cache state.

.github/actions/setup/directories/action.yml

@@ -42,6 +42,7 @@ runs:
       run: |
         mkdir -p ${{ inputs.srcdir }}
         mkdir -p ${{ inputs.builddir }}
+        mkdir -p ${{ inputs.srcdir }}/.downloaded-cache
 
     # Did you know that actions/checkout works without git(1)?  We are
     # checking that here.