Support for lock file v4

[nazar-pc]

Right now lock files with version = 4 created by nightly versions fail like this:

error: not found: Couldn't load Cargo.lock
Caused by:
  -> I/O operation failed: parse error: parse error: invalid Cargo.lock format version: `4`

Would be great to support this, especially if this is not a big lift.

[tarcieri]

Dup of rustsec/rustsec#1249

[Shnatsel]

Reopening now that cargo audit supports lockfile v4, but this action reportedly doesn't.

[nazar-pc]

It kind of does and doesn't at the same time, which I believe is caused by this line:

audit-check/src/main.ts

Line 18 in 69366f3

await cargo.findOrInstall('cargo-audit');

It will not try to upgrade cargo-audit when older version that doesn't support v4 lock file is currently installed. This doesn't affect official runners in most cases, but it may affect self-hosted runners that do not clean the state or simply workflows that happen to have cargo-audit of older version installed for whatever reason.