Consider avoiding using "which" to validate prereqs

[mellinoe]

which is not universally available, and is not even installed by default in some cases (docker images). We should consider using a more universally available method for checking. Answers online recommend using command or type to do this.

[mellinoe]

This is what I'm referring to, for clarity.

# resolve python-version to use
if [ "$PYTHON" == "" ] ; then
    if which python >/dev/null 2>&1
    then
       PYTHON=python
    elif which python2 >/dev/null 2>&1
    then
       PYTHON=python2
...

[adityamandaleeka]

I've opened a PR to address this.

[jkotas]

Rolled back in dotnet/coreclr#7025