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

**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
```