ci: unbreak cargo-nextest

Upstream cargo-nextest shipped a broken version v0.9.56 [0], which is now
yanked, and superseded by v0.9.57. The GHA installer, however, hasn't
updated yet [1], so using a temporary workaround to unbreak CI.
The cargo-nextest maintainer has been lighting fast and extremely
communicative, so let's check back on the GHA issue later on, at which
point we can drop this empty file.

[skip ci]

[0] nextest-rs/nextest#926
[1] taiki-e/install-action#178