Make master branch test use prereleases

This should really have been here since day 1. It's quite common for
the various EPICS python modules to have prerelease versions at the tip
of their main.

Author: AlexanderWells-diamond

.github/workflows/code.yml

@@ -191,8 +191,8 @@ jobs:
 
       - name: Install master versions
         run: |
-          pip install git+https://github.com/epics-base/p4p.git git+https://github.com/epics-base/pvxs.git git+https://github.com/epics-base/epicscorelibs.git
-          pip install git+https://github.com/DiamondLightSource/aioca git+https://github.com/DiamondLightSource/cothread
+          pip install --pre git+https://github.com/epics-base/p4p.git git+https://github.com/epics-base/pvxs.git git+https://github.com/epics-base/epicscorelibs.git
+          pip install --pre git+https://github.com/DiamondLightSource/aioca git+https://github.com/DiamondLightSource/cothread
           pip install -e .[dev]
           pip freeze