gecko: wait for rust before swiching llvm* next time

Cross-language LTO in www/firefox requires devel/llvm* and bundled
LLVM in lang/rust to be of the same major version. Adjust the comment
to delay future bumps but until LTO is enabled desync can still happen.

PR: 255359
Suggested by: dim

(cherry picked from commit b1c90afe23f93698805a22802f2dd02acbe4488a)