Rollup merge of #106886 - dtolnay:fastinstall, r=Mark-Simulacrum

Rollup merge of #106886 - dtolnay:fastinstall, r=Mark-Simulacrum

Make stage2 rustdoc and proc-macro-srv disableable in x.py install

Rustdoc will build if `[build] tools = ["rustdoc"]` is set, and rust-analyzer-proc-macro-srv will build if `[build] tools = ["rust-analyzer"]` is set.

On my machine skipping these tools speeds up `x.py install` from 7m15s to 6m08s (0m43s for rustdoc and 0m24s for rust-analyzer-proc-macro-srv). This is a significant speedup, since I never use rust-analyzer-proc-macro-srv, and I practically never need to use a custom build of rustdoc.