package/nodejs: more python3 fixes

nodejs' build system can not work with python3.

Complement the existing patch to use a variable instead of hard-coding
python.

Fixes:
    http://autobuild.buildroot.org/results/406/406291bf7c61a80ed4a8d14d9a3887f51a1b2254/
    http://autobuild.buildroot.org/results/ff9/ff97f452b875d8b1fb78561609742a233b80e0d3/
    http://autobuild.buildroot.org/results/7ad/7adb96b45cb539c46940908d4ce9ca78b6a84d81/


    ...

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>