pager: set LESS=FRSX also on Windows

pager: set LESS=FRSX also on Windows

Previously, this environment variable was set in the pager_preexec
callback, which is conditionally-compiled only on Unix, because it is not,
and cannot be, called on Windows.

With this patch the env member of struct child_process is used to set
the environment variable, which also works on Windows.

Noticed by Alexey Borzenkov.

Signed-off-by: Johannes Sixt <redacted>
Signed-off-by: Junio C Hamano <redacted>