ci: Use a prefix for the meson build in the windows image

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer-rs/-/merge_requests/1084>

ci/run_windows_tests.ps1

@@ -1,6 +1,6 @@
 # Add the precompiled gst binaries to the path
-$env:Path += ';C:\bin\'
-$env:PKG_CONFIG_PATH = "C:/lib/pkgconfig"
+$env:Path += ';C:/gst-install/bin\'
+$env:PKG_CONFIG_PATH = "C:/gst-install/lib/pkgconfig"
 
 # List of all the crates we want to build
 # We need to do this manually to avoid trying

ci/windows-docker/Dockerfile

@@ -10,7 +10,7 @@ ARG DEFAULT_BRANCH="main"
 ARG RUST_VERSION="invalid"
 
 RUN choco install -y pkgconfiglite
-ENV PKG_CONFIG_PATH="C:/lib/pkgconfig"
+ENV PKG_CONFIG_PATH="C:\gst-install\lib\pkgconfig"
 
 COPY install_gst.ps1 install_gtk.ps1 C:\
 RUN C:\install_gst.ps1

ci/windows-docker/install_gst.ps1

@@ -26,7 +26,8 @@ if (!$?) {
   Exit 1
 }
 
-$env:MESON_ARGS = "-Dglib:installed_tests=false " +
+$env:MESON_ARGS = "--prefix=C:\gst-install\ " +
+    "-Dglib:installed_tests=false " +
     "-Dlibnice:tests=disabled " +
     "-Dlibnice:examples=disabled " +
     "-Dffmpeg:tests=disabled " +

ci/windows-docker/install_gtk.ps1

@@ -11,7 +11,7 @@ if (!$?) {
 
 Set-Location C:\gtk
 
-$env:MESON_ARGS = ""
+$env:MESON_ARGS = "--prefix=C:\gst-install\"
 
 Write-Output "Building gtk"
 cmd.exe /C "C:\BuildTools\Common7\Tools\VsDevCmd.bat -host_arch=amd64 -arch=amd64 && meson _build $env:MESON_ARGS && meson compile -C _build && ninja -C _build install"