scripts: west: runners: stm32cubeprogrammer: enable exec from PATH

Allows to use the stm32cubeprogrammer runner on Linux to be executed
from any installation directory as long as the executable is found
in PATH.

Until now on Linux the programmer could only be used if it either has
been installed in the default location or if the executable path was
passed via the cli parameter.

Signed-off-by: Thomas Stranger <thomas.stranger@outlook.com>