Revert "mb: Allow custom build dirs for the isolate-everything run mode"

This reverts commit 75004f6b767c69f166011a3aec6ca0d3b6ec69f8.

Reason for revert: to fix tree closure:
https://ci.chromium.org/ui/p/chromium/builders/ci/Deterministic%20Linux/46720/infra

Original change's description:
> mb: Allow custom build dirs for the isolate-everything run mode
>
> "GetConfig()" mostly just wraps "Lookup()", but is smart enough to
> gracefully re-use a pre-existing build dir. This can be leveraged by
> the UTR in instances where the UTR hasn't used mb.py to gen.
> Practically that's:
> - when the user skips "compile" mode
> - when the user overrides with some custom GN args
>
> So this allows "mb.py isolate-everything" to be used in both of those
> cases by preventing it from over-writing the local build dir.
>
> Bug: 41492686
> Change-Id: I150672865d5cc333b946603cac61716281f643a5
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5411040
> Reviewed-by: Struan Shrimpton <sshrimp@google.com>
> Commit-Queue: Ben Pastene <bpastene@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#1281261}

No-Presubmit: true
No-Tree-Checks: true
No-try: true
Bug: 41492686
Change-Id: Ib841f66c730f13d7667f9a798eb79f350879a4f5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5414142
Reviewed-by: Struan Shrimpton <sshrimp@google.com>
Auto-Submit: Ben Pastene <bpastene@chromium.org>
Commit-Queue: Struan Shrimpton <sshrimp@google.com>
Cr-Commit-Position: refs/heads/main@{#1281285}