Rollup merge of #131095 - GuillaumeGomez:switch-to-env-variables, r=n…

…otriddle

Use environment variables instead of command line arguments for merged doctests

Fixes [#130796](rust-lang/rust#130796).

Before merging this one, let's first ensure it has a smaller impact compared to command line arguments (results can be seen [here](rust-lang/rust#130285)). I'll start a crater run once CI passed.

cc ``@ehuss``
r? ``@notriddle``