fix: use $() to precompute clang version

[JLHwung]

Fix clang_version does not expand as expected. This fix is sub-product of previous curl compression patch (#1550).

One can verify the correctness of the patch from the following snippet

#!/bin/bash

fxy() {
    echo "${1#v}" and "${2#v}"
}

g() {
    echo "h"
}

echo $(fxy g h)
echo $(fxy "$(g)" h)

[ljharb]

Great catch!

I don't think this is tested, so this fix is good as-is.

[JLHwung]

You are welcome. I really stuck for a while on curl_use_compression before I figure out that nvm_curl_version is not expanded at all 😅. so I knew there is similar situations on clang_version.