Use relative ref for bakery-build-pr.yml

Now that bakery-build-pr.yml exists on main (merged in #442),
the relative ref resolves correctly. This matches the pattern
used by bakery-build.yml and bakery-build-native.yml, and lets
PRs that modify bakery-build-pr.yml test their own changes.

Author: bschwedler

.github/workflows/ci.yml

@@ -152,7 +152,7 @@ jobs:
     permissions:
       contents: read
       packages: write
-    uses: posit-dev/images-shared/.github/workflows/bakery-build-pr.yml@main
+    uses: "./.github/workflows/bakery-build-pr.yml"
     with:
       context: "./posit-bakery/test/resources/multiplatform/"
       dev-versions: include