The scripts path was looked up passing explicitly the scheme to be
used using "nt" on Windows and "posix_prefix" everywhere else.
However, when the isolated build environment is created, packages are
installed using the default scheme for the platform. On most platforms
this works because normally "nt" and "posix_prefix" are the default
schemes.

However, Debian customizes sysconfig to use a "posix_local" scheme by
default and under this scheme the scripts path does not match the one
of the "posix_prefix" scheme. This results in scripts installed as
part of the build dependencies not to be found during the build, as
reported here mesonbuild/meson-python#109
and here https://bugs.debian.org/1019293.

The problem can be solved omitting to specify a scheme when looking up
the scripts path. To future proof the path lookup, use the "venv"
scheme if available as done in pypa#11598. For uniformity use similar
functions as used to lookup the library paths.

Since there's not equivalent of get_isolated_environment_bin_path for
the distutils backend, the additional declaretion in the sysconfig
backend is unnecessary.

The list is *almost* sorted alphabetically so let's make it fully so.