[Chapel Merge] Update rules that update chpl-venv's install to blow away the dir

Branch: refs/heads/main
Revision: 0c650fe6c6191ee1099dd1f5f9823221c2164ace
Author: bradcray
Link: Update rules that update chpl-venv's install to blow away the dir by bradcray · Pull Request #28746 · chapel-lang/chapel · GitHub
Log Message:
Update rules that update chpl-venv's install to blow away the dir (#28746)

[reviewed by @jabraham17]

Without this — as I understand it — what would happen is that when
requirements were updated, the new requirements would be added to the
existing directory, and things like chpldoc would use the first version
found, which wouldn't necessarily be the latest. This resulted in things
like chpldoc using version 0.0.40 of the Chapel Sphinx domain when I had
version 0.0.41 installed and listed in the requirements file, requiring
developers to manually remove the directory to get the latest version /
not get testing failures, which felt like a common annoyance.

Compare: Comparing 06a29130006e9aaf1724ddd8fd4ad1f0b261fa7e...07c39bf61042e1280e3c76d5a81e8a082075ec75 · chapel-lang/chapel · GitHub

Diff:
M third-party/chpl-venv/Makefile
https://github.com/chapel-lang/chapel/pull/28746.diff