update toolchain to nightly-2026-04-08 #1613

Job	Run time
Difftest (macOS-latest)	12m 15s
Test (ubuntu-latest)	11m 11s
Difftest (ubuntu-latest)	14m 18s
Test (macOS-latest)	14m 56s
Compiletest (macOS-latest)	8m 19s
Difftest (windows-latest)	21m 37s
Compiletest (windows-latest)	17m 29s
Compiletest (ubuntu-latest)	7m 28s
Android (ubuntu-latest, aarch64-linux-android)	8m 23s
Lint	5m 45s
cargo-deny	1m 11s
Test (windows-latest)	22m 53s
test_success	2s
	2h 25m 47s

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