scons: Fixing build errors with scons 4.0.1 and 4.1.0

SCons failed to find m5 module while loading m5.util.terminal
from site_scons/gem5_scons/util.py.

This results in the current version of gem5 stable failed to
build with scons 4.0.1 and 4.1.0.

The nature of the bug and the explaination for the fix can be
found here,
https://gem5-review.googlesource.com/c/public/gem5/+/38616

Jira: https://gem5.atlassian.net/browse/GEM5-916

Change-Id: I3817f39ebc3021fb6fc89bcd09a96999f8ca2841
Signed-off-by: Hoa Nguyen <hoanguyen@ucdavis.edu>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/41594
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>

SConstruct

@@ -139,7 +139,7 @@ if GetOption('no_lto') and GetOption('force_lto'):
 #
 ########################################################################
 
-main = Environment()
+main = Environment(tools=['default', 'git'])
 
 from gem5_scons.util import get_termcap
 termcap = get_termcap()

site_scons/site_tools/default.py

@@ -78,15 +78,9 @@ def common_config(env):
     # as well
     env.AppendENVPath('PYTHONPATH', extra_python_paths)
 
-gem5_tool_list = [
-    'git',
-]
-
 def generate(env):
     common_config(env)
     SCons.Tool.default.generate(env)
-    for tool in gem5_tool_list:
-        SCons.Tool.Tool(tool)(env)
 
 def exists(env):
     return 1