Detect the system on windows instead of reading Windows_NT

ci/lib.sh works out OS only when the caller has not set one:

    if [[ ! ${OS-} ]]; then OS=$(os); fi

Windows predefines OS as Windows_NT in every process environment, so on a
windows shell that test always finds a value, os() is never called, and OS
stays Windows_NT for the whole build. Every question the build then asks about
the system gets an answer it does not understand: which bin scripts to fix up,
whether to build packages, what the release archive is called.

os() already handles this platform -- `cygwin* | mingw*` answers windows -- it
just never gets the chance. Windows_NT is not a name this build knows, so it is
not treated as one a caller chose.

Measured on a windows shell, sourcing the file three ways:

    OS unset            -> windows     (was Windows_NT)
    OS=Windows_NT       -> windows     (was Windows_NT)
    OS=linux            -> linux       (unchanged)

so an explicit choice still wins, which is the point of the check. Nothing
changes anywhere else: no other platform sets OS, so the added test is never
reached off windows.

Author: denusklo

ci/lib.sh

@@ -49,7 +49,10 @@ if [[ ! ${ARCH-} ]]; then
   export ARCH
 fi
 
-if [[ ! ${OS-} ]]; then
+# Windows predefines OS as Windows_NT for every process, so on a windows shell
+# the check below would always find a value and never call os(). That is not a
+# name this build knows, so it does not count as one the caller chose.
+if [[ ! ${OS-} || ${OS-} == "Windows_NT" ]]; then
   OS=$(os)
   export OS
 fi