sbang: put sbang in the install_tree (#11598)

`sbang` is not always accessible to users of packages, e.g., if Spack
is installed in someone's home directory and they deploy software
for others.  Avoid this by:

1. Always installing the `sbang` script in the `install_tree`
2. Relocating binaries to point to the copy in the `install_tree` 
   and not the one in the Spack installation.

This PR also:
- ensures that `sbang` is reinstalled if it is modified in Spack
- adds tests
- updates the way `gobject-introspection` patches Makefiles
   to support `sbang`

Co-authored-by: Todd Gamblin <tgamblin@llnl.gov>

var/spack/repos/builtin/packages/gobject-introspection/package.py

@@ -4,7 +4,7 @@
 # SPDX-License-Identifier: (Apache-2.0 OR MIT)
 
 from spack import *
-from spack.paths import spack_root
+import spack.hooks.sbang as sbang
 
 
 class GobjectIntrospection(Package):
@@ -72,7 +72,7 @@ def install(self, spec, prefix):
         make("install")
 
     def setup_build_environment(self, env):
-        env.set('SPACK_SBANG', "%s/bin/sbang" % spack_root)
+        env.set('SPACK_SBANG', sbang.sbang_install_path())
 
     @property
     def parallel(self):