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:

• 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
• Building twice on the same machine yields the same binaries
• Reproduce between Github Actions and locally built
• Still runs well
• Document how to build and how to compare

[JohnAZoidberg]

Works