source-dist: Building with hadrian requires alex/happy on path

When making the source dist we take an effort to generate all the alex/happy files to put in the source dist so that the user doesn't need to use alex or happy to generate them.

• The build currently fails if ALEX="" and HAPPY="" because they are not optional builders and we try to always pass --with-alex and --with-happy when configuring.
• Then there are also build-tool-depends which cabal configure fails to satisfy when building with hadrian, but are needed for when building with cabal-install.

We should also modify the test-bootstrap job so that it tests the build in an environment without setting ALEX and HAPPY.