abuild rootbld: disable real chown calls in fakeroot

By default, fakeroot forwards chown system calls to the libc and ignores EPERM errors. Unfortunately, when fakeroot is used inside a restricted bubblewrap environment (as created by abuild rootbld), bubblewrap intercepts these system calls and returns EINVAL. The EINVAL return value is not masked by fakeroot and returned to the caller, thereby causing failures of mv(1), install(1), … which use chown system calls internally.

Setting the FAKEROOTDONTTRYCHOWN environment variable prevents the chown systemcalls to be performed in the first place.

Fixes #10021 (closed)