File permissions changing inside the sandbox

The issue

When writing the integration test (in bst-external) for the dpkg-build plugin, in order for the test to pass, debian/rules must be built.

Now, locally executing bst build dpkg-build/dpkg-build-test.bst resulted in a successful build. However, when this command is launched in the sandbox, using the following test:

https://gitlab.com/BuildStream/bst-external/blob/convert-to-new-style-integration-tests/tests/dpkg-build.py#L14

The test fails due to a permissions change of the debian/rules file, see failed job here:

https://gitlab.com/BuildStream/bst-external/-/jobs/57647277

A quick fix

This can be fixed by using chmod +x <file> to explicitly change the permission of the file and prepending this as a build command, like so: https://gitlab.com/BuildStream/bst-external/blob/convert-to-new-style-integration-tests/tests/project/elements/dpkg-build/dpkg-build-test.bst#L15

The problem

Obviously this solution is not practical for a project with many .bst files.