gapic-generator: use `install_unittest_dependencies` in `prerelease_deps` and `core_deps_from_source`

[ohmayr]

The prerelease_deps and core_deps_from_source sessions in the GAPIC generator's noxfile.py.j2 template currently use hardcoded boilerplate (session.install("-e", ".")) to install the local package and test dependencies. This boilerplate bypasses optional package extras (UNIT_TEST_EXTRAS), causing continuous CI jobs to fail for hybrid packages (GAPIC_COMBO) that rely on extra test dependencies.

Rather than duplicating dependency installation logic across multiple sessions, prerelease_deps and core_deps_from_source should call install_unittest_dependencies(session).