Use correct OS variable in download URL (#10323)

Addresses an issue found by @jonvanausdeln in the download URL for the
WSL binary; it uses `Linux` (platform) instead of `linux` (server OS).

Author: jmcphers

extensions/open-remote-wsl/src/serverSetup.ts

@@ -274,7 +274,7 @@ if [[ ! -d $SERVER_DIR ]]; then
     fi
 fi
 
-SERVER_DOWNLOAD_URL="$(echo "${serverDownloadUrlTemplate.replace(/\$\{/g, '\\${')}" | sed "s/\\\${quality}/$DISTRO_QUALITY/g" | sed "s/\\\${version}/$DISTRO_VERSION/g" | sed "s/\\\${commit}/$DISTRO_COMMIT/g" | sed "s/\\\${os}/$PLATFORM/g" | sed "s/\\\${arch}/$SERVER_ARCH/g" | sed "s/\\\${arch-long}/$ARCH_LONG/g" | sed "s/\\\${release}/$DISTRO_VSCODIUM_RELEASE/g")"
+SERVER_DOWNLOAD_URL="$(echo "${serverDownloadUrlTemplate.replace(/\$\{/g, '\\${')}" | sed "s/\\\${quality}/$DISTRO_QUALITY/g" | sed "s/\\\${version}/$DISTRO_VERSION/g" | sed "s/\\\${commit}/$DISTRO_COMMIT/g" | sed "s/\\\${os}/$SERVER_OS/g" | sed "s/\\\${arch}/$SERVER_ARCH/g" | sed "s/\\\${arch-long}/$ARCH_LONG/g" | sed "s/\\\${release}/$DISTRO_VSCODIUM_RELEASE/g")"
 
 
 # Check if server script is already installed