scons: Fix checking the --install-hooks option in the "git" tool.

The option should be checked with 'install_hooks' and not
'install-hooks', even though the option is '--install-hooks' to the
user.

This bug was due to an incorrect review suggestion from me on this
original change:

https://gem5-review.googlesource.com/c/public/gem5/+/50410

JIRA: https://gem5.atlassian.net/browse/GEM5-1091

Change-Id: Ifb4526307bea015de1fe73d3d685477be711b7cb
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/50628
Maintainer: Gabe Black <gabe.black@gmail.com>
Reviewed-by: Eric Ye <ericye@google.com>
Reviewed-by: Hoa Nguyen <hoanguyen@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>

site_scons/site_tools/git.py

@@ -100,7 +100,7 @@ def install_style_hooks(env):
         return
 
     print(git_style_message, end=' ')
-    if SCons.Script.GetOption('install-hooks'):
+    if SCons.Script.GetOption('install_hooks'):
         print("Installing revision control hooks automatically.")
     else:
         try: