mail/thunderbird-esr: fix build on 2026Q2

The build failed with (and probably because of) the old rust in
Quarterly, as some of the cargo-checksum.json files in
comm/third_party/rust (the thunderbird part) referenced .gitmodules
files which are not present in the release tarball.
Stripping out these references (most easily with sed) fixes the build.

This is a stop-gap fix in 2026Q2 only - the build on main is fine,
and the 2026Q2 will be closed in two weeks anyway.