GHA: stop deleting curl in non-native workflows

GHA: stop deleting curl in non-native workflows

We do it in Cirrus CI, but for some platforms it's not possible to
delete it and tests work anyway.

The test runner also runs `../src/curl` by default, which is always the
one freshly built. The runner may also need the system curl to talk to
APIs when needed.

Also:
- stop setting `CURL` env. This isn't picked up by the runners,
  and works out of the box anyway.
- quote an option just in case.

Follow-up to 90e644f944969bb11c6448bf50c6d441b5c0b1e6 #13583
Closes #13765