Set GIT_CEILING_DIRECTORIES during prepare/build/check/package

Many build systems expect or check if they are run in a git repository, and either get information from it, like the version of the project or even modify it (installing hooks, like husky annoyingly does). As we get the source code from archives, there is never an upstream repository involved, and as a result, aports is being used.

Git has an environment variable called GIT_CEILING_DIRECTORIES:

This should be a colon-separated list of absolute paths. If set, it is a list of directories that Git should not chdir up into while looking for a repository directory

So if we set something like:

export GIT_CEILING_DIRECTORIES=$STARTDIR

then git will never see aports as a repository while building projects, preventing incorrect information leaking into projects and projects like husky damaging aports.

As abuild itself needs to get information from aports, we cannot blankly set it somewhere global. It should also be in affect when a user manually runs abuild build for example.