Upgrade Rust toolchain to nightly-2026-04-18 #1666

Job	Run time
cargo-deny	1m 10s
Compiletest (windows-latest)	19m 20s
Compiletest (ubuntu-latest)	7m 51s
cargo-gpu-os (windows-latest)	14m 42s
Lint	5m 49s
Test (macOS-latest)	15m 5s
cargo-gpu-os (macos-latest)	8m 20s
Test (ubuntu-latest)	12m 5s
Compiletest (macOS-latest)	9m 43s
Android (ubuntu-latest, aarch64-linux-android)	8m 22s
Test (windows-latest)	22m 11s
Difftest (macOS-latest)	12m 24s
Difftest (ubuntu-latest)	15m 28s
cargo-gpu-os (ubuntu-latest)	7m 29s
Difftest (windows-latest)	21m 53s
cargo-gpu-backwards-compat (f79c4181a5dc2d37303947b113f190930c6c1ce6, =0.30.7)	7m 3s
cargo-gpu-backwards-compat (30896871ba00e668029ccb724f1438202b284708)	7m 16s
cargo-gpu-backwards-compat (eea8998df9dc2fd8e7a65c5b5b7ae20c238a665a, 0.29.3)	6m 40s
cargo-gpu-backwards-compat (0.9.0, 0.24.2)	6m 31s
cargo-gpu-backwards-compat (e767f24f2565baf1a71bbaf84d453d181cab2417)	6m 58s
cargo-gpu-backwards-compat (2326b87fe1542eeb898065e36ac949307b55386d, =0.30.7)	6m 43s
cargo-gpu-backwards-compat (877bd8697a15f3e6d09446a5e1807e6237ca1dac)	7m 11s
test_success	2s
	3h 50m 16s

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