chore: CodeBuild build errors when branch is not 'master' (#10868)

The problem is that the shell conditional added returns a non-zero exit
code when the branch is not 'master'.

Switch strategies so that the environment variable comparison is
actually done at deploy time and sets a final environment variable.

Corresponding change: https://github.com/aws/cdk-ops/pull/758

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

buildspec.yaml

@@ -18,9 +18,7 @@ phases:
       - /bin/bash ./fetch-dotnet-snk.sh
   build:
     commands:
-      # we bump here so that our master version won't be identical to the latest published version during tests.
-      # otherwise this causes problems with verdaccio mirroring.
-      - '[ ${GIT_BRANCH} = ${REGRESSION_TESTS_BRANCH} ] && /bin/bash ./bump-candidate.sh'
+      - 'if ${BUMP_CANDIDATE:-false}; then /bin/bash ./bump-candidate.sh; fi'
       - /bin/bash ./scripts/align-version.sh
       - /bin/bash ./build.sh
   post_build:

bump-candidate.sh → scripts/bump-candidate.sh

@@ -20,4 +20,4 @@ version=${1:-minor}
 
 echo "Starting candidate ${version} version bump"
 
-npx standard-version --release-as ${version} --prerelease=rc --skip.commit --skip.changelog
+npx standard-version --release-as ${version} --prerelease=rc --skip.commit --skip.changelog