test if a PR produces the same binaries

[lsf37]

It's sometimes useful to know whether a change still produces the same binaries or not. E.g. when source code is just re-arranged or config options changes/renamed etc.

Since seL4 builds are reproducible, this is testable, and could be an optional GH action triggered by a label.

[lsf37]

This also helps to pin down test flakiness. If the binary has not changed, but a test fails, it must be something in the test environment that changed (hardware, timing, etc)