Disable generating on Ubuntu image

.github/workflows/lockfile.yml

@@ -30,14 +30,19 @@ jobs:
       run:
         shell: ${{ matrix.sys.shell }}
 
+    # Building on an Ubuntu image is disabled until such time as it is proveably possible to
+    # build python-rtmidi v1.5.x (with or without Poetry).
+    #
+    # @note At the time of writing, python-rtmidi 1.5.x hasn't arrived in Ubuntu's (or Debian's)
+    #       repositories.
     strategy:
       fail-fast: false
       matrix:
         sys:
         - os: windows-latest
           shell: msys2 {0}
-        - os: ubuntu-latest
-          shell: bash
+        #- os: ubuntu-latest
+        #  shell: bash
         - os: macos-latest
           shell: bash