tools: add the bootstrapped _depot_tools dir to the PATH (for V8 tests)

[DeeDeeG]

Short summary:

This script needs to execute a copy of ninja in order to complete.

The script downloads Google's depot_tools early on. (depot_tools includes a copy of the ninja binary.) I propose to add those to the PATH, so that running ninja later in the script succeeds for users at home running this outside of CI.

Background, context, more details:

The make-v8.sh script (used by the test-v8 Makefile target) implicitly requires a separate copy of ninja on the PATH, even though it bootstraps a copy of ninja for you (that it proceeds to not use).

This Pull Request adds the _depot_tools dir bootstrapped during this script to the PATH, before ninja is executed, taking advantage of the fact that _depot_tools conveniently includes a copy of ninja. This makes for one less system requirement to be able to finish the test-v8 Makefile target.

(I am not sure why this script chooses to add ~/_depot_tools to the PATH, as at least oiutside of CI on my personal machine, that directory never exists on my machine during the entire test-v8 Makefile target from beginning to end. But I interpret that as evidence that depot_tools is already meant to be on the PATH in some form or other.)

I was looking into documenting the system requirements needed to finish the test-v8 Makefile target. (I wanted to document that this is now the only part of this repository (that I know of) that still requires Python 2 after #36691, given that some Python scripts in the upstream depot_tools and v8 repositories still aren't Python 3 compatible).

When I was testing that Makefile target locally, I noticed something a bit strange (to my eyes) about an implicit ninja requirement:

• The test-v8 Makefile target implicitly expects ninja on the PATH in order to complete
• The test-v8 Makefile target ends up bootstrapping a copy of depot_tools before that, at deps/v8/_depot_tools
  • This includes a copy of ninja
• A seemingly arbitrary path ~/_depot_tools is added to the PATH, but this isn't ensured to exist/isn't bootstrapped by the test-v8 Makefile target
  • (To my eyes, this path is arbitrary/maybe not useful, since it would be sheer coincidence if the user has depot_tools there, but I left it in case CI needs it... It should now be fully redundant as of this PR, so I think the script could probably skip adding that location to the PATH. Unless CI needs a custom depot_tools there with modifications? It works for me locally with upstream depot_tools, so I can't think of why that would be needed.)
• The deps/v8/_depot_tools which was just bootstrapped is not put on the PATH
  • I feel like it should be? This gives us ninja, and we went out of our way to get it, so I think we should take advantage.

It feels like the bootstrapped _depot_tools is supposed to be on the PATH so we can use its copy of ninja
to complete the rest of building V8. Rather than documenting this quirky and redundant (maybe accidental) ninja requirement, I thought I'd have a go at making it unnecessary to manually ensure ninja is on the PATH, by using the copy that was just bootstrapped.

An alternative might be simply directly executing the deps/v8/_depot_tools/ninja binary without altering the PATH whatsoever. But I haven't yet tested whether that works. Works for me, not sure whether it works just as well in CI.

[DeeDeeG]

I did some tiny updates:

• Stopped having the script add ~/_depot_tools to the PATH, because this probably doesn't exist for most users. (As discussed above.)
• Made the code finding _depot_tools simpler/smaller, and in the process, made the logic locating _depot_tools happen relative to the current dir deps/v8 rather than the repo root, to better match nearby lines of the script that are also relative to the current dir (deps/v8), not relative to the repo root.

(So, it's one less line of code, and slightly more robust to potential repo refactoring or moving folders around, or at least that's the idea.)

I opted not to remove depot_tools from PATH for those commands at the end, for two reasons:

• I know CI does have depot_tools on the PATH, and I would like users running this outside of CI to get similar results as what will happen in CI.
• V8 tells you to have depot_tools on the PATH in their official documentation for building V8
  • https://v8.dev/docs/source-code
  • https://v8.dev/docs/build

[richardlau]

V8 CI: https://ci.nodejs.org/job/node-test-commit-v8-linux/3952/

[jasnell]

Landed in 6e3ce65