Use makefile nightly in diplomat-coverage

[sffc]

Follow-up to #5283

[sffc]

This works locally; why doesn't it work on CI?

error: environment variable `ICU4X_DIPLOMAT_COVERAGE_NIGHTLY` not defined at compile time
   --> tools/make/diplomat-coverage/src/main.rs:102:35
    |
102 |                 .args(["install", std::env!("ICU4X_DIPLOMAT_COVERAGE_NIGHTLY")])
    |                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = help: use `std::env::var("ICU4X_DIPLOMAT_COVERAGE_NIGHTLY")` to read the variable at run time
    = note: this error originates in the macro `std::env` (in Nightly builds, run with -Z macro-backtrace for more info)

error: environment variable `ICU4X_DIPLOMAT_COVERAGE_NIGHTLY` not defined at compile time
   --> tools/make/diplomat-coverage/src/main.rs:108:21
    |
108 |                     std::env!("ICU4X_DIPLOMAT_COVERAGE_NIGHTLY"),
    |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = help: use `std::env::var("ICU4X_DIPLOMAT_COVERAGE_NIGHTLY")` to read the variable at run time
    = note: this error originates in the macro `std::env` (in Nightly builds, run with -Z macro-backtrace for more info)

[robertbastian]

I don't know why it doesn't work in CI, but even locally this shouldn't work, because the rustdoc-types dependency is tightly coupled to nightly ranges.

[robertbastian]

.

[sffc]

The CI failure is before it gets around to rustdoc-types.

Should we leave this one alone and close the PR?

[Manishearth]

We should try to update to a newer (but pinned) nightly here if possible but I don't think it's super important