Files in archives generated by ApiGetFlowFilesArchiveHandler have incorrect "Date modified" metadata

[simstoykov]

Environment

• How did you install GRR? https://grr-doc.readthedocs.io/en/latest/developing-grr/setting-up-dev-env.html
• What GRR version are you running?: version.ini says: major = 3, minor = 4, revision = 2, release = 8
• What operating system does the GRR server run on?: latest gLinux
• What operating system does the affected GRR client run on, if applicable?: latest gLinux

Describe the issue
I am utilizing the GetFilesArchiveMappings flow functionality, which adds archive-generation capabilities to a flow for collected files on the server. The implementation seems to be here:
https://github.com/google/grr/blob/master/grr/server/grr_response_server/gui/api_plugins/flow.py#L638

The issue is that "Date modified" metadata of files in the archive seems to be set incorrectly to "Jan 1, 1980, 12:00 AM":

Output from ls:

simstoykov@penguin:~/C_4d10af5f6739f473_F1A7616E8AECA377_OsqueryFlow/osquery_collected_files/usr/local/google/home/simstoykov$ ls -lh
total 24K
-rw------- 1 simstoykov simstoykov 1.0K Jan  1  1980 fr1.txt
-rw------- 1 simstoykov simstoykov 1.0K Jan  1  1980 hard_test_file.txt
-rw------- 1 simstoykov simstoykov 1.0K Jan  1  1980 names_generated.txt
-rw------- 1 simstoykov simstoykov 1.0K Jan  1  1980 names_original.txt
-rw------- 1 simstoykov simstoykov 1.0K Jan  1  1980 sym_test_file.txt
-rw------- 1 simstoykov simstoykov 1.0K Jan  1  1980 test_file.txt