spack bootstrap dev: detect git as an external (#39417)

#36770 added git as a dependency to `setuptools-scm`. This in turn makes `git` a
transitive dependency for our bootstrapping process. 

Since `git` may take a long time to build, and is found on most systems, try to 
detect it as an external.

lib/spack/spack/bootstrap/core.py

@@ -480,11 +480,18 @@ def _add_externals_if_missing() -> None:
         spack.repo.PATH.get_pkg_class("bison"),
         # GnuPG
         spack.repo.PATH.get_pkg_class("gawk"),
+        # develop deps
+        spack.repo.PATH.get_pkg_class("git"),
     ]
     if IS_WINDOWS:
         search_list.append(spack.repo.PATH.get_pkg_class("winbison"))
-    detected_packages = spack.detection.by_executable(search_list)
-    spack.detection.update_configuration(detected_packages, scope="bootstrap")
+    externals = spack.detection.by_executable(search_list)
+    # System git is typically deprecated, so mark as non-buildable to force it as external
+    non_buildable_externals = {k: externals.pop(k) for k in ("git",) if k in externals}
+    spack.detection.update_configuration(externals, scope="bootstrap", buildable=True)
+    spack.detection.update_configuration(
+        non_buildable_externals, scope="bootstrap", buildable=False
+    )
 
 
 def clingo_root_spec() -> str:

lib/spack/spack/bootstrap/environment.py

@@ -23,6 +23,7 @@
 
 from ._common import _root_spec
 from .config import root_path, spec_for_current_python, store_path
+from .core import _add_externals_if_missing
 
 
 class BootstrapEnvironment(spack.environment.Environment):
@@ -185,6 +186,7 @@ def pytest_root_spec() -> str:
 
 def ensure_environment_dependencies() -> None:
     """Ensure Spack dependencies from the bootstrap environment are installed and ready to use"""
+    _add_externals_if_missing()
     with BootstrapEnvironment() as env:
         env.update_installations()
         env.update_syspath_and_environ()