[Fix] use command to bypass curl/wget shell functions and aliases

zsh (and interactive bash with `expand_aliases`) bakes a preexisting `curl` alias into nvm's function bodies at source time,
and shell functions named `curl`/`wget` shadow the binaries at call time - either one breaks downloads.
Prefixing invocations with `command` bypasses both: here, `nvm_download`'s dispatch, `nvm_curl_version`, `nvm_curl_libz_support`, and the wget branch of `nvm_get_latest`;
the remaining bare `curl` invocations in `nvm_get_latest` and the install script are prefixed in a followup commit.

The tests that previously mocked curl/wget as shell functions now install fake executables on PATH instead,
via a shared `make_fake_curl` helper in `test/common.sh`,
and a new test asserts the bypass.

Refs #2923

install.sh

@@ -119,7 +119,7 @@ nvm_download() {
                             -e 's/-o /-O /' \
                             -e 's/-C - /-c /')
     # shellcheck disable=SC2086
-    eval wget $ARGS
+    eval command wget $ARGS
   fi
 }