Fix CI ccache for library and restrict size

[mikee47]

Fix CI library caching

#2865 broken library CI because:

• ccache initialisation commands never get executed to set ccache directory
• library ccache key must have toolchain in it

Fixed by removing the ccache commands from script and updating workflow env instead.
Just requires call to clear stats before building.

NB. This leads to a fair bit of duplication in all 3 workflow files. This is the simplest I can come up with for now.

Simplify workflows

Don't need the Configure environment step, just add values to env.

Use explicit ccache path

In testing was getting failures saving cache due to ccache not found.
Use explicit absolute path to ccache directory.

Restrict ccache size

The cache for each job will increase over time to hit 500MB.
Over 40 jobs that's 20GB and will cause cache thrashing.

Easy fix: ask ccache to evict anything older than 4 hours from ccache.
Do this before printing stats so we can see the resulting size.