Some versions of `/bin/sh`, e.g. the one on FreeBSD, do not propagate
variables set on a command through shell functions. This results in
`FISH_GETTEXT_EXTRACTION_FILE` being set in the `cargo` function, but
not for the actual `cargo` process spawned from the function, which
breaks our localization scripts and tests. Exporting the variable
prevents that.
Fixes#11896Closes#11899