submodule-sync: make fewer path assumptions (#90)

Before this commit,
`per-submodule-build-matrix.sh` assumed that the
top level of the repo it was syncing was at
`../../../../../`, which happened to be true for
all of our repos. As of
reboot-dev/mono@323ce51
however, that is no longer true.

This commit makes the script look up the top-level
of its containing repo dynamically, with its only
assumption being that the script is being run from
a submodule itself - which still holds.

TESTED: checked that the `submodule-sync` workflow
passes again for the `reboot-dev/respect` repo
where it was previously broken.

.github/workflows/scripts/per-submodule-build-matrix.sh

@@ -1,12 +1,29 @@
 #!/bin/bash
 
+# We'll print some debug information along the way.
+echo "Started in working directory '$(pwd)'..."
+
+# Assumption: this script is being run from _inside_ a submodule, but is
+# supposed to act on the repo that _contains_ that submodule. Change our working
+# directory accordingly.
+cd "$(git rev-parse --show-toplevel)/.."
+
+gitmodules_path="$(git rev-parse --show-toplevel)/.gitmodules"
+echo "Assumed relevant '.gitmodules' file is '$gitmodules_path'"
+
 # Update references.
 git submodule update --recursive --remote
 
 # The following condition is needed to set required outputs.
 # The step generates one output: `path_matrix`.
 # `path_matrix` output contains list of all submodules within a repo.
 # The flag is used by the main build job.
-echo "path_matrix=[$(git config --file ../../../../../.gitmodules --get-regexp path | \
-awk '{ print $2 }' | \
-awk '{ printf "%s\"%s\"", (NR==1?"":", "), $0 } END{ print "" }')]" >> $GITHUB_OUTPUT
+output=$( \
+    echo "path_matrix=[$(git config --file "$gitmodules_path" --get-regexp path | \
+      awk '{ print $2 }' | \
+      awk '{ printf "%s\"%s\"", (NR==1?"":", "), $0 } END{ print "" }')]" \
+)
+echo "####"
+echo $output
+echo "####"
+echo $output >> $GITHUB_OUTPUT