Forbid unprefixed SHAs for toolchains

[lqd]

This PR forbids unprefixed SHAs for toolchain arguments, to avoid crater hanging when they appear.

I'm not sure this is fully valid though: I don't know if SHAs could appear in a Toolchain's name (and RustwideToolchain::dist()) without being an error 😓. This commonly happens with rustup-toolchain-install-master but I don't know if a similar situation could arise within common crater usage.

CI will surely fail: some of the used dependencies are broken on nightly right now, and clippy emits warnings -- both issues are fixed in #722.

_{(this is for @oli-obk and for @compiler-errors)}

[Mark-Simulacrum]

@bors r+

This seems reasonable to me.

[bors]

:pushpin: Commit aedec6d80d9fdba33160d5b81e5fd93b624b4ca0 has been approved by Mark-Simulacrum

It is now in the queue for this repository.

[bors]

:hourglass: Testing commit aedec6d80d9fdba33160d5b81e5fd93b624b4ca0 with merge a00f6b3739991d6b337130311bc769f17c59777f...

[bors]

:broken_heart: Test failed - checks-actions

[Mark-Simulacrum]

@bors r+

[bors]

:bulb: This pull request was already approved, no need to approve it again.

• This pull request previously failed. You should add more commits to fix the bug, or use retry to trigger a build again.

[bors]

:pushpin: Commit aedec6d80d9fdba33160d5b81e5fd93b624b4ca0 has been approved by Mark-Simulacrum

It is now in the queue for this repository.

[bors]

:hourglass: Testing commit aedec6d80d9fdba33160d5b81e5fd93b624b4ca0 with merge f2a10b4a604952d9434cecdbd42f46efd6b24757...

[bors]

:sunny: Test successful - checks-actions Approved by: Mark-Simulacrum Pushing f2a10b4a604952d9434cecdbd42f46efd6b24757 to master...

[bors]

:sunny: Test successful - checks-actions Approved by: Mark-Simulacrum Pushing f2a10b4a604952d9434cecdbd42f46efd6b24757 to master...

[bors]

:eyes: Test was successful, but fast-forwarding failed: 422 Changes must be made through a pull request.