[Fix] nvm_download, nvm_get_latest, install: only select a downloader that exists as an executable

`nvm_has` matches shell functions and aliases,
but downloads now run via `command`, which skips them
- a `curl` shell function with no curl binary on the PATH would select the curl path and fail with exit 127,
instead of falling back to an available wget executable.
The new `nvm_has_executable` helper resolves names the same way `command` does, so downloader selection and execution agree.

test/fast/Unit tests/nvm install --offline

@@ -30,9 +30,10 @@ nvm_echo "$CAPTURED_STDERR" | nvm_grep -q "${EXPECTED_ERR}" \
 
 # --offline should not require curl or wget
 nvm_has() { return 1; }
+nvm_has_executable() { return 1; }
 try_err nvm install --offline 999.999.999
 # Should fail with "not found" not "nvm needs curl or wget"
 nvm_echo "$CAPTURED_STDERR" | nvm_grep -q "curl or wget" \
   && die "nvm install --offline should not require curl or wget"
 alias nvm_has='\nvm_has'
-unset -f nvm_has
+unset -f nvm_has nvm_has_executable