Drop use of 'make avail' following https://github.com/cc65/cc65/pull/1994

cc65's 'make avail' now fails unless all targets were built.
Create links explicitly for ca65/ld65 only.

action.yml

@@ -5,6 +5,8 @@ runs:
   steps:
     - run: >
         git clone https://github.com/cc65/cc65 /tmp/cc65 &&
-        sudo make -C /tmp/cc65 ca65 ld65 avail &&
+        sudo make -C /tmp/cc65 ca65 ld65 &&
+        sudo ln -s /tmp/cc65/bin/ca65 /usr/local/bin/ca65 &&
+        sudo ln -s /tmp/cc65/bin/ld65 /usr/local/bin/ld65 &&
         ca65 --version
       shell: bash