WIP: Try to make the firmware build reproducible

[JohnAZoidberg]

Changes:

• Use --remap-path-prefix
• Enable strip=true
• Pin rust version in rust-toolchain.toml

Status:

• [x] Use --remap-path-prefix. Currently need to manually patch the binary
• [ ] See if strip = true does anything useful for us. I can see it strips the ELF binary but UF2 conversion already gets rid of most(all?) of that
• [x] Building twice on the same machine yields the same binaries
• [x] Reproduce between Github Actions and locally built
• [x] Still runs well
• [ ] Document how to build and how to compare

[JohnAZoidberg]

Works