scons: Don't fail builds if TERM is not set

Bug: 204817626

Test: Scons build
Change-Id: I154c69503eaededda353d6b2925553b6cecb07eb
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/52304
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

util/m5/SConstruct

@@ -94,7 +94,8 @@ else:
 # Propogate the environment's PATH setting.
 main['ENV']['PATH'] = os.environ['PATH']
 # Pass through terminal information to, for instance, enable color output.
-main['ENV']['TERM'] = os.environ['TERM']
+if 'TERM' in os.environ:
+    main['ENV']['TERM'] = os.environ['TERM']
 # Pass through the java CLASSPATH (if it exists) so we can find libraries.
 main['ENV']['CLASSPATH'] = os.environ.get('CLASSPATH', '')