`which` utility is not shipped by default for example on recent Arch
Linux and then any steps relying on its presence fails, like for example
following Python3 prereq build check:
$ python3 --version
Python 3.9.1
$ make
/bin/sh: line 1: which: command not found
/bin/sh: line 1: which: command not found
/bin/sh: line 1: which: command not found
...
Checking 'python3'... failed.
...
Fix this by switching to Bash builtin `command` which should provide
same functionality.
Fixes: FS#3525
Signed-off-by: Petr Štetiar <ynezz@true.cz>
c7aec47e5e
