Auto merge of #116449 - Kobzol:bootstrap-rustdoc-shim, r=petrochenkov

Pass flags to `rustdoc` shim without env. vars

Discussed here: rust-lang/rust#116448 (comment). Since it was not really documented why these flags were passed through the shim, I guess that the only way to find out if it's really needed... is to remove it :)

r? `@petrochenkov`

Author: bors