CI Linux: prevent mkappimage auto upload

mkappimage now tries to automatically upload the asset if it detects
running in GitHub CI, which is not the behavior we perhaps want because
it deletes and recreates our _continuous_ release.

See following failed run:
https://github.com/CESNET/UltraGrid/actions/runs/5575039257/jobs/10185765784

+ fixed `wget` presence check in `create-appimage.sh`

data/scripts/Linux-AppImage/create-appimage.sh

@@ -81,7 +81,7 @@ fi
 
 add_fonts
 
-if command wget >/dev/null && wget -V | grep -q https; then
+if command -v wget >/dev/null && wget -V | grep -q https; then
         dl() {
                 wget -O - ${GITHUB_TOKEN+--header "Authorization: token $GITHUB_TOKEN"} "$1"
         }
@@ -167,8 +167,9 @@ UPDATE_INFORMATION=
 if [ $# -ge 1 ]; then
         UPDATE_INFORMATION="-u zsync|$1"
 fi
-# shellcheck disable=SC2086 # word spliting of $UPDATE_INFORMATION is requested behavior
-$mkappimage $UPDATE_INFORMATION $APPDIR
+# shellcheck disable=SC1007,SC2086 # word spliting of
+# $UPDATE_INFORMATION is a requested behavior
+GITHUB_TOKEN= $mkappimage $UPDATE_INFORMATION $APPDIR
 
 rm -rf $APPDIR tmpinstall
 )