Update CI to use configured pgx version

[eeeebbbbrrrr]

Right now, plrust's CI installs cargo-pgx from pgx' git repo, off its develop branch.

Instead, CI should peek inside plrust/Cargo.toml and pluck out the pgx version we're using, and simply execute cargo install cargo-pgx --version $TheVersionFromCargoToml.

Assuming CI's existing caching features do what they claim, this will allow subsequent CI runs that all use the same pgx version to skip installing cargo-pgx again, possibly saving ~4 minutes per job.

[BradyBonnette]

Yeah good idea. As mentioned in discord, I am fairly certain installing cargo-pgx from the develop branch was a relic of the past. Now that things are solidifying a bit, we should do this.

[eeeebbbbrrrr]

Right now we run cargo install cargo-pgx ... --force. We don't want --force anymore if we're going to expect cargo install to skip installing the same version.

[BradyBonnette]

Got it.

[BradyBonnette]

@eeeebbbbrrrr #182 has been merged in, so this issue can be closed