build: remove .git folders when testing V8

When running `make test-v8` V8's `gclient sync` converts folders
under `deps/v8/third_party` into git repositories. Unfortunately
the files that were checked in under `deps/v8/third_party/zlib`
have been modified from the upstream Chromium repository (some
files have been deleted and there are whitespace differences in
some of the files that were kept) so whenever the Node.js source
tree is hard reset/checked out `gclient sync` notices there are
unstaged changes as the files in the Node.js source tree do not
match those of the upstream Chromium third party zlib commit.

Signed-off-by: Richard Lau <riclau@uk.ibm.com>

PR-URL: https://github.com/nodejs/node/pull/32877
Refs: https://github.com/nodejs/build/issues/2256


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Matheus Marchini <mat@mmarchini.me>
Reviewed-By: Anna Henningsen <anna@addaleax.net>