Windows: Fix external detection for service accounts (#37293)

Prior to this PR, the HOMEDRIVE environment variable was used to
detect what drive we are operating in. This variable is not available
for service account logins (like what is used for CI), so switch to
extracting the drive from PROGRAMFILES (which is more-widely defined).

lib/spack/spack/detection/common.py

@@ -218,8 +218,10 @@ def update_configuration(detected_packages, scope=None, buildable=True):
 
 
 def _windows_drive():
-    """Return Windows drive string"""
-    return os.environ["HOMEDRIVE"]
+    """Return Windows drive string extracted from PROGRAMFILES
+    env var, which is garunteed to be defined for all logins"""
+    drive = re.match(r"([a-zA-Z]:)", os.environ["PROGRAMFILES"]).group(1)
+    return drive
 
 
 class WindowsCompilerExternalPaths(object):