Add custom version for verilator --version packaging (#4954)

docs/CONTRIBUTORS

@@ -136,6 +136,7 @@ Mostafa Gamal
 Nandu Raj
 Nathan Kohagen
 Nathan Myers
+Nolan Poe
 Oleh Maksymenko
 Patrick Stewart
 Paul Swirhun

src/config_rev

@@ -22,7 +22,11 @@ Args = parser.parse_args()
 
 os.chdir(Args.directory)
 
-rev = 'UNKNOWN_REV'
+if 'VERILATOR_SRC_VERSION' in os.environ:
+    rev = os.environ['VERILATOR_SRC_VERSION']
+else:
+    rev = 'UNKNOWN_REV'
+
 data = os.popen('git describe').read()
 
 match = re.search(r'^(v[0-9].*)', data)