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

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)`.

Open the GAPIC generator's noxfile.py.j2 template and locate the prerelease_deps and core_deps_from_source sessions, then inspect the existing install_unittest_dependencies(session) helper. Replace the duplicated installation logic so both sessions use the helper, preserving UNIT_TEST_EXTRAS support for GAPIC_COMBO packages; done means the sessions no longer use the hardcoded install boilerplate.

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