Reduce duplication in Task::run

[jyn514]

Previously, this would create a new TaskCtx for every task that needed
to run. Now it only constructs it once.

This also had the most confusing error message I've ever had the
misfortune to see in Rust:

error[E0308]: mismatched types
   --> src/runner/tasks.rs:255:38
    |
255 |         test::run_test(action, &ctx, test)?;
    |                                      ^^^^ one type is more general than the other
    |
    = note: expected fn pointer `for<'r, 's, 't0, 't1, 't2> fn(&'r runner::tasks::TaskCtx<'s, DB>, &'t0 rustwide::Build<'t1>, &'t2 std::collections::HashSet<cargo_metadata::PackageId>) -> std::result::Result<_, _>`
               found fn pointer `fn(&runner::tasks::TaskCtx<'_, DB>, &rustwide::Build<'_>, &std::collections::HashSet<cargo_metadata::PackageId>) -> std::result::Result<_, _>`

The fix was to change

        let (action, test, toolchain, quiet): (_, fn(_, _, _) -> _, _, _) = match self.step {

to

        let (action, test, toolchain, quiet): (_, fn(&TaskCtx<_>, &Build, &_) -> _, _, _) = match self.step {

I have no idea why.

[pietroalbini]

Thanks!

@bors r+

[bors]

📌 Commit 8efe38a has been approved by pietroalbini

[bors]

⌛ Testing commit 8efe38a with merge dcc5456...

[bors]

☀️ Test successful - checks-actions
Approved by: pietroalbini
Pushing dcc5456 to master...