ci: pin nightly toolchain when using minimal-versions (#742) #1315

Job	Run time
build with default features	28s
build with MSRV (1.66.1)	1m 38s
build with no_std (ristretto255)	34s
Clippy	29s
build with no_std (ed25519)	21s
build with no_std (p256)	26s
Check Rust doc	32s
build with no_std (secp256k1)	27s
build with no_std (rerandomized)	19s
Rustfmt	14s
Check if automatically generated code is up to date	22s
test on beta	2m 13s
actionlint	7s
	8m 10s

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