maint/CICD ~ fix coverage build failures by pinning to known-working nightly toolchain
maint/CICD ~ fix coverage build failures by pinning to known-working nightly toolchain

## [why]

Code coverage must currently use some unstable features in nightly rust builds. The
nightly builds are, by definition, unstable and subject to frequent breaking changes.
To prevent CI build breakage, the toolchain is pinned to a specific known working set.

Note: (maint!) this will require periodic review until code coverage is more fully
implemented/integrated into Rust and moved into the stable channel.

- refs: <https://github.com/mozilla/grcov/issues/427>, <https://github.com/newsboat/newsboat/issues/916>

uutils/coreutils,uutils/coreutils,uutils/coreutils