Create a different latest nightly symlink for each branch

Affects nightly builds uploaded to the OSUOSL server. Currently we
create a single 'latest' symlink on each platform, e.g. on macOS:

    MuseScoreNightly-latest-x86_64.dmg

Now we will create one for each branch:

    MuseScoreNightly-latest-master-x86_64.dmg
    MuseScoreNightly-latest-4.0.2-x86_64.dmg

This helps to avoid confusion, and also avoids the race condition where
the 'latest' symlink sometimes points to a master nightly and sometimes
to a 4.0.2 nightly depending on the time of day.

On Linux, for similar reasons, a separate zsync automatic updates file
will be created for each branch.

The current "branchless" symlink and zync files will remain unmodified
on the server until they are eventually deleted by the same code that
is used to delete old nightly files on each upload.

build/ci/linux/package.sh

@@ -82,7 +82,7 @@ if [ "$PACKTYPE" == "appimage" ]; then
     # https://github.com/AppImage/AppImageSpec/blob/master/draft.md#update-information
     case "${BUILD_MODE}" in
     "stable_build")  export UPDATE_INFORMATION="gh-releases-zsync|musescore|MuseScore|latest|MuseScore-*x86_64.AppImage.zsync";;
-    "nightly_build") export UPDATE_INFORMATION="zsync|https://ftp.osuosl.org/pub/musescore-nightlies/linux/${MAJOR_VERSION}x/nightly/MuseScoreNightly-latest-x86_64.AppImage.zsync";;
+    "nightly_build") export UPDATE_INFORMATION="zsync|https://ftp.osuosl.org/pub/musescore-nightlies/linux/${MAJOR_VERSION}x/nightly/MuseScoreNightly-latest-${BUILD_BRANCH}-x86_64.AppImage.zsync";;
     *) unset UPDATE_INFORMATION;; # disable updates for other build modes
     esac
 

build/ci/tools/osuosl/publish.sh

@@ -76,8 +76,11 @@ chmod 600 $SSH_KEY
 
 FTP_PATH=${OS}/${MAJOR_VERSION}x/${BUILD_DIR}
 
-file_extension="${ARTIFACT_NAME##*.}"
-LATEST_NAME="MuseScoreNightly-latest-x86_64.${file_extension}"
+if [ "$BUILD_MODE" == "nightly_build" ]; then
+    file_extension="${ARTIFACT_NAME##*.}"
+    BUILD_BRANCH=$(cat $ARTIFACTS_DIR/env/build_branch.env)
+    LATEST_NAME="MuseScoreNightly-latest-${BUILD_BRANCH}-x86_64.${file_extension}"
+fi
 
 echo "Copy ${ARTIFACTS_DIR}/${ARTIFACT_NAME} to $FTP_PATH"
 scp -oStrictHostKeyChecking=no -C -i $SSH_KEY $ARTIFACTS_DIR/$ARTIFACT_NAME musescore-nightlies@ftp-osl.osuosl.org:~/ftp/$FTP_PATH