[infra] Copy environment to ensure debian-package can find ninja

The copy was accidentally removed when removing bot.py.

TEST=BUILDBOT_BUILDERNAME="debianpackage-linux-be" ./tools/run_debian_build.sh

Change-Id: I9264824246a96a83f171602571d2073243776d64
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/170686
Reviewed-by: William Hesse <whesse@google.com>

tools/bots/linux_distribution_support.py

@@ -45,7 +45,8 @@ def CreateDartTestFile(tempdir):
 def Run(command):
     print "Running: %s" % ' '.join(command)
     sys.stdout.flush()
-    no_color_env = {'TERM': 'nocolor'}
+    no_color_env = dict(os.environ)
+    no_color_env['TERM'] = 'nocolor'
     exit_code = subprocess.call(command, env=no_color_env)
     if exit_code != 0:
         raise OSError(exit_code)