ci: Improve MinGW/MSYS Makefiles mingw.osdn.io job environments

Set `PATH`, `MSYSTEM`, and `MAKE_MODE` environment variables the way the
MinGW/MSYS 1.0 distribution from `mingw.osdn.io` does in its bash prompt.

.gitlab/ci/env_mingw_osdn_io_mingw_makefiles.ps1

@@ -1,3 +1 @@
-$pwdpath = $pwd.Path
-& "$pwsh" -File ".gitlab/ci/mingw.ps1"
-Set-Item -Force -Path "env:PATH" -Value "$pwdpath\.gitlab\mingw\bin;$env:PATH"
+. .gitlab/ci/mingw-env.ps1

.gitlab/ci/env_mingw_osdn_io_msys_makefiles.ps1

@@ -1,5 +1 @@
-$pwdpath = $pwd.Path
-& "$pwsh" -File ".gitlab/ci/mingw.ps1"
-Set-Item -Force -Path "env:PATH" -Value "$pwdpath\.gitlab\mingw\msys\1.0\bin;$pwdpath\.gitlab\mingw\bin;$env:PATH"
-$env:MSYSTEM = 'MINGW32'
-$env:MAKE_MODE = 'unix'
+. .gitlab/ci/mingw-env.ps1

.gitlab/ci/mingw-env.ps1

@@ -0,0 +1,5 @@
+$pwdpath = $pwd.Path
+& "$pwsh" -File ".gitlab/ci/mingw.ps1"
+Set-Item -Force -Path "env:PATH" -Value "$pwdpath\.gitlab\mingw\bin;$pwdpath\.gitlab\mingw\msys\1.0\bin;$env:PATH"
+$env:MSYSTEM = 'MINGW32'
+$env:MAKE_MODE = 'unix'