Tag builds done through `alr build` with unique build string

[mosteo]

This may help down the line with reports of people using development versions, to uniquely identify them.

A script is invoked before build to patch in the commit (and an extra flag if the working folder is dirty).

After the build, "_or_later" is added to the build string, so we have a lower bound, but we are not mislead by manual builds that don't trigger the patching (via pure gprbuild).

So, the new versions would look like 2.0-dev+abcd1234 or 2.0-dev+abcd1234_or_later.

Since this triggers rebuilding a bunch of files if using alr build to build alr itself, it can be disabled by exporting ALR_VERSION_DONT_PATCH in the environment to avoid those rebuilds during development.

[mosteo]

I will rewrite the python part in Ada to avoid having a hidden dependency on Python, so I'm leaving this unmerged for now.