Speed up PR Validation

[simongdavies]

We should try and speed up PR validation, currently its taking over 30 minutes, we should look at if we need to do all the validation we are currently doing or if more can be done once or in parallel e.g.

• Benchmarks now take around 12 minutes or so but don't need t run sequentially with build/test
• We build the guests in every validation run rather than building once up front

[jsturtevant]

both of these suggestions sound reasonable and would improve validation time dramatically.

We could also introduce caching to the rust builds and maybe run some work to https://github.com/hyperlight-dev/hyperlight/actions/workflows/RustNightly.yml