chore: fix master build by skipping optional release notes copy (#17154)

For releases (v1 and v2), we need to create the release notes (as part of the
build) and then copy them to the dist/ directory (as part of pack). However, for
`master` builds, we don't create the release notes because the BUMP_CANDIDATE
builds reference a non-existent release version. Assuming the presence of the
release notes in pack is causing the `master` pipeline to fail.


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pack.sh

@@ -94,7 +94,10 @@ HERE
 
 # copy CHANGELOG.md and RELEASE_NOTES.md to dist/ for github releases
 cp ${changelog_file} ${distdir}/CHANGELOG.md
-cp RELEASE_NOTES.md ${distdir}/RELEASE_NOTES.md
+# Release notes are not available for bump candidate builds.
+if ! ${BUMP_CANDIDATE:-false}; then
+  cp RELEASE_NOTES.md ${distdir}/RELEASE_NOTES.md
+fi
 
 # defensive: make sure our artifacts don't use the version marker (this means
 # that "pack" will always fails when building in a dev environment)