lang/rust-nightly: no longer builds after r479877

mv: rename /wrkdirs/usr/ports/lang/rust-nightly/work/rustc-nightly-src/src/vendor/backtrace-sys/src/libbacktrace/config/libtool.m4 to /wrkdirs/usr/ports/lang/rust-nightly/work/rustc-nightly-src/src/vendor/backtrace-sys/src/libbacktrace/config/libtool.m4.dont-fix: No such file or directory

• Error code 1

As the port appears to be a staging ground for future updates don't
deprecate yet.

PR: 229826
Reported by: pkg-fallout