build: replace which with Bash command built-in

build: replace which with Bash command built-in

`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>