Auto merge of #127799 - Kobzol:bootstrap-cmd-refactor-7, r=onur-ozkan

Bootstrap command refactoring: make command output API more bulletproof (step 7)

Continuation of rust-lang/rust#127680.

This PR modifies the API of running commands to make it more explicit when a command is expected to produce programmatically handled output. Now if you call just `run`, you cannot access the stdout/stderr by accident, because it will not be returned to the caller.

This API change might be seen as overkill, let me know what do you think. In any case, I'd like to land the second commit, to make it harder to accidentally read stdout/stderr of commands that did not capture output (now you'd get an empty string as a result, but you should probably get a panic instead, if you try to read uncaptured stdout/stderr).

Tracking issue: rust-lang/rust#126819

r? `@onur-ozkan`

try-job: x86_64-msvc