ci: use trybuild branch which strips rust-src lines

[davidhewitt]

This PR adds a temporary patch to our trybuild dependency which strips out rust standard library src lines from error messages. This removes our need to install rust-src in CI, hopefully resolving our rustup issue which has blocked CI.

See dtolnay/trybuild#247

[davidhewitt]

How do fellow contributors feel about depending on a patch of try build for the moment, to unblock CI? If the trybuild PR is rejected we can switch to a different solution, though I don't think any are as tidy as this one.

[adamreichold]

How do fellow contributors feel about depending on a patch of try build for the moment, to unblock CI? If the trybuild PR is rejected we can switch to a different solution, though I don't think any are as tidy as this one.

Not great but still better than being unable to merge anything.

As a wild guess, did you try to delete rust-toolchain.toml so that we only install the component upon explicit request? IIRC, I saw those errors earlier and resolved them back then by explicitly installing the component upfront instead when automatically request via that file.

[davidhewitt]

I'll push a PR and see what happens...

[davidhewitt]

Well, it looks like #3575 will go green, so let's start with that.