Provides an entry point that builds and returns a combined LH + RH keyboard
firmware when provided a keymap via a POST body.
Wraps compilation with ccache, and includes a pre-warmed cache of the build in /tmp/ccache.
To maximize chance of a direct cache hit, changes the lambda driver to always build in /tmp/build.
some back of the envelope measurements (2012 xeon e3-1230v2, nixos)
clean build, no cache -> 21.308
clean build, cache -> 7.145
modified keymap, clean build, cache -> 12.127
