Make preinstall work again (fixes docker build)

[wvengen]

Building the Docker image failed because Busybox doesn't support the ps argument used in preinstall.

Also, the yarn check always failed actually, because $PPID is the shell, not yarn. This fixed both.

(a failed and succeeded build)

[huksley]

Another possible solution is to install procps alpine package before running bin/build.

[wvengen]

@huksley thanks, that may indeed provide a better ps. Nevertheless, the check would still be broken, as the ppid isn't (or not guarenteed to be) the package manager, but a shell spawned by it. The approach of this PR is used in other software (and on SO).

[tlrobinson]

@wvengen Thanks, I wasn't aware of npm_execpath, this seems much more reliable.