Make snapcraft work with ccache

[Olivier Paroz]

Despite having a properly configured ccache using $PATH, parts don't seem to be using it when compiling code.
It should be automatically picked up and used.

A simple test is to build a snap using a simple C++ example. Compiling by hand uses ccache, compiling uses snapcraft doesn't.

```
$ whereis g++
g++: /usr/bin/g++ /usr/lib/ccache/g++ /usr/share/man/man1/g++.1.gz
$ which g++
/usr/lib/ccache/g++

```

[林博仁(Buo-ren Lin) (brlin)]

As a workaround you may use the ccache remote part:
https://forum.snapcraft.io/t/speed-up-snap-building/7979

[Mike Miller (mtmiller)]

I have run into this, installing ccache in the host and building a snap with --destructive. I think this is because snapcraft prepends directories to PATH, so 'gcc' is found under /snap/.../usr/bin, not under /usr/lib/ccache.

A workaround that works for me is setting the environment variables CC="ccache gcc" and CXX="ccache g++" in the build environment.