build: don't depend on libidn2

The recent libcurl update in commit 0919f390802f ("third_party: update
libcurl from 8.8.0 to 8.10.1") reveals an old problem in our libcurl's
build script: it attempts to disable the libidn2 dependency using the
`-DHAVE_LIBIDN2=OFF` option instead of `-DUSE_LIBIDN2=OFF`.

It doesn't work after a recent curl building machinery change, see
https://github.com/curl/curl/pull/14555

This commit fixes the problem by using `-DUSE_LIBIDN2=OFF`.

Reported in #10621

NO_DOC=build mechanics fixup
NO_CHANGELOG=fixes a non-released commit
NO_TEST=verified locally, seems to need an extra work on a separate CI
        workflow with a particular building environment; no time for
        that tonight, sorry

(cherry picked from commit a4b4158f2196442308aba7df8f8e21b98b5319fc)