ci: Bump nightly toolchain

[Rahix]

Use the same version that is now used by avr-hal.

[Rahix]

Hm, so we are running into Rahix/avr-hal#537 here as well now. I'm considering whether going "nuclear" and just pinning proc-macro2 for avr-device-macros directly might be the best solution:

diff --git a/macros/Cargo.toml b/macros/Cargo.toml
index 617e80b..5c3ece9 100644
--- a/macros/Cargo.toml
+++ b/macros/Cargo.toml
@@ -17,7 +17,7 @@ proc-macro = true
 
 [dependencies]
 quote = "1.0.7"
-proc-macro2 = "1.0.19"
+proc-macro2 = "=1.0.79"
 
 [dependencies.syn]
 version = "1.0.35"

Please voice your opinions...

[Rahix]

A downside of pinning in avr-device-macros is that downstream users cannot easily override it, I think. This means that when people want to switch to a different toolchain version, they can't adjust the proc-macro2 pin to work alongside it... Unfortunate :(