Use cargo check consistently in prepare

[cycraig]

Description of change

Small PR to change cargo sqlx prepare to always use cargo check.

Currently, prepare invokes cargo rustc by default but switches to check when the --merged flag is used. This can lead to confusion since certain flags passed to cargo check will be rejected by cargo rustc when running prepare.

The current behaviour also contradicts the sqlx-cli README, which states:

In order for sqlx to be able to find queries behind certain feature flags you need to turn them on by passing arguments to rustc.

This is how you would turn all targets and features on.

cargo sqlx prepare -- --all-targets --all-features

Which actually fails with the following error:

error: extra arguments to `rustc` can only be passed to one target, consider filtering
the package by passing, e.g., `--lib` or `--bin NAME` to specify a single target
error: `cargo check` failed with status: exit code: 101   

But passes with --merged, regardless of whether the current directory is a workspace root or a sub-crate.
cargo sqlx prepare --merged -- --all-targets --all-features

This PR ensures it works with or without --merged.

When the working directory is a crate in a workspace, it still only builds the queries for it and not the entire workspace.
Is there any reason to retain the behaviour of using rustc directly? Edit: does this negatively affect plans for #1706?

This arose from the following comment thread: #2069 (comment)

Alternative: update the documentation/README.md to specify --merged must be used to combine the queries of multiple targets, even when in a single crate.

P.S. sorry for spamming PRs.

Type of change

Bug fix to match the intended behaviour, but might technically be a breaking change as it changes which flags are accepted or not.

How the change has been tested

cd sqlx-cli && cargo test and running cargo sqlx prepare -- --all-targets --all-features in a sample project.

[abonander]

This PR has conflicts after merging #2069.

[cycraig]

Thanks for the ping; rebased the changes.