Respect $SOURCE_DATE_EPOCH in generate_bram_types_sim.py

The variable is set in build environments to allow the build to be
reproducible (identical result in independent builds),
see https://reproducible-builds.org/docs/source-date-epoch/.
The code snippet is based on the sample from that page too.

I'm working on build reproducibility of Fedora packages, and this
patch fixes an issue observed in test rebuilds.

techlibs/quicklogic/qlf_k6n10f/generate_bram_types_sim.py

@@ -1,10 +1,17 @@
+import os
 import sys
+import time
 from datetime import datetime, timezone
 
 def generate(filename):
+	build_date = datetime.fromtimestamp(
+		int(os.environ.get('SOURCE_DATE_EPOCH', time.time())),
+		tz=timezone.utc,
+	)
+
 	with open(filename, "w") as f:
 		f.write("// **AUTOGENERATED FILE** **DO NOT EDIT**\n")
-		f.write(f"// Generated by {sys.argv[0]} at {datetime.now(timezone.utc)}\n")
+		f.write(f"// Generated by {sys.argv[0]} at {build_date}\n")
 
 		f.write("`timescale 1ns /10ps\n")
 		for a_width in [1,2,4,9,18,36]: