git's export-subst is a reproducibility problem

ab180083 added testing/web-platform/tests/tools/third_party/attrs/.git_archival.txt.

It's a template file that is filled thanks to the export-subst flag set in testing/web-platform/tests/tools/third_party/attrs/.gitattributes.

However, it's a possible source for reproducibility problems. At least, it was in MB 15.0a1 (see tor-browser-build#41440 (comment 3230364)).

Generally speaking, we might want to ignore the export-subst when running git archive.