archive: fix stored paths when building under MSYS

When building CArchive package under MSYS, os.path.sep is Unix-style
forward clash separator that is incompatible with bootloader (which
on Windows expects back slash separators). Therefore, if we are on
Windows and os.path.sep is forward slash, os.path.normpath() in
CTOC.add() needs to be followed by explicit replacement of forward
slashes with back slashes.

Otherwise, bootloader fails to extract nested files from archive in
onefile builds that are created in MSYS environment.

PyInstaller/archive/writers.py

@@ -35,7 +35,7 @@
     fake_pyc_timestamp
 from PyInstaller.loader.pyimod02_archive import PYZ_TYPE_MODULE, PYZ_TYPE_PKG, \
     PYZ_TYPE_DATA, PYZ_TYPE_NSPKG
-from ..compat import BYTECODE_MAGIC, is_py37
+from ..compat import BYTECODE_MAGIC, is_py37, is_win
 
 
 class ArchiveWriter(object):
@@ -283,6 +283,10 @@ def add(self, dpos, dlen, ulen, flag, typcd, nm):
         # slashes '\\' since on Windows the bootloader works only with back
         # slashes.
         nm = os.path.normpath(nm)
+        if is_win and os.path.sep == '/':
+            # When building under MSYS, the above path normalization
+            # uses Unix-style separators, so replace them manually.
+            nm = nm.replace(os.path.sep, '\\')
         self.data.append((dpos, dlen, ulen, flag, typcd, nm))