Configure `project_name` for GoReleaser

This causes the format of the checksum file generated during the release
to change from `flux2_*_checksums.txt` to `flux_*_checksums.txt`.

The configuration change is made through `project_name` and not via the
`checksum.name_template` setting, because a single checksum file is
generated during the release process.

The download and/or installation script in `install/flux.sh` has been
adapted to assume the new filename starting with MINOR version `0.6.0`.

Signed-off-by: Hidde Beydals <hello@hidde.co>