Re-enable building on Ubuntu

(Apparently there's been some improvements to the project's own build flow)

(Also, 1.5.x is in Debian without needing patched, so...)

.github/workflows/lockfile.yml

@@ -30,19 +30,14 @@ 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