upgrade rust-toolchain.toml to reference nightly-07-08

[mutantbob]

There is a bug in LLVM regarding MOVW being translated into a pair of 8-bit register moves which could malfunction if the register pairs overlapped. This was discovered while troubleshooting the ST7789 crate ( almindor/st7789#19 ) and was reported to the rust toolchain as rust-lang/rust#98167 . The fix appears to be part of nightly-07-08, and also sorted out some strange behavior in ufmt .

I recommend that you give nightly-07-08 a try, and if you do not find any showstoppers then make it the default channel in rust-toolchain.toml .

[Rahix]

Already resolved now with 6494918.