Use "command -v" instead of "which" to detect python executable (#1419)

On a Linux distro that doesn't have the `which` program installed we're
getting the following error:

    $ ./emsdk install latest
    ./emsdk: line 39: exec: python: not found

It's failing to detect the installed `python3` and falls back to using
`python`, but this distro doesn't provide a python -> python3 symlink so
we fail.

Fix this by using `command -v` instead which is a POSIX standard.
The same change went into emscripten a couple years ago:
https://github.com/emscripten-core/emscripten/pull/15071

emsdk

@@ -26,7 +26,7 @@ fi
 # If bundled python is not found, look for `python3` in PATH.  This is especially important on macOS (See:
 # https://github.com/emscripten-core/emsdk/pull/273)
 if [ -z "$EMSDK_PYTHON" ]; then
-  if PYTHON3="$(which python3 2>/dev/null)"; then
+  if PYTHON3="$(command -v python3 2>/dev/null)"; then
     EMSDK_PYTHON=$PYTHON3
   fi
 fi