Fix check-git-dependencies for indented source-repository-package stanzas

The grep and sed patterns were anchored to the start of line
(^source-repository-package), but the stanza is now indented inside
an `if impl(ghc >= 9.12)` block. Allow leading whitespace so the
array marker `-` is correctly inserted into the YAML, preventing the
downstream jq parse error.

Author: Jimbo4350

.github/bin/check-git-dependencies

@@ -23,7 +23,7 @@ commits () {
   done
 }
 
-grep '\(^source-repository-package\|^ *location:\|^ *tag:\)' cabal.project | sed 's|^source-repository-package|-|g' | \
+grep '\(^ *source-repository-package\|^ *location:\|^ *tag:\)' cabal.project | sed 's|^ *source-repository-package|-|g' | \
   yq eval -P -o=json \
   > tmp/repositories.json