Use environment secrets for Codecov token #3444

Job	Run time
alternative_targets (ubuntu-latest, wasm32-unknown-emscripten)	26s
features_check (ubuntu-latest)	3m 52s
alternative_targets (ubuntu-latest, aarch64-apple-ios)	14s
no_std (ubuntu-latest, thumbv6m-none-eabi)	19s
rust_msrv (ubuntu-latest)	56s
alternative_targets (ubuntu-latest, aarch64-linux-android)	8s
no_std (ubuntu-latest, x86_64-fortanix-unknown-sgx)	10s
rust_versions (ubuntu-latest, stable)	57s
rust_versions (ubuntu-latest, beta)	1m 8s
timezones (ubuntu-latest, ACST-9:30)	1m 19s
rust_versions (ubuntu-latest, nightly)	48s
timezones (ubuntu-latest, EST4)	1m 18s
timezones (ubuntu-latest, UTC0)	1m 30s
timezones (ubuntu-latest, Asia/Katmandu)	1m 21s
timezones (macos-latest, ACST-9:30)	34s
timezones (macos-latest, EST4)	37s
timezones (macos-latest, UTC0)	45s
timezones (macos-latest, Asia/Katmandu)	36s
timezones (windows-latest, ACST-9:30)	2m 5s
timezones (windows-latest, EST4)	2m 30s
check-docs	22s
timezones (windows-latest, UTC0)	2m 46s
timezones (windows-latest, Asia/Katmandu)	2m 44s
cross-targets (x86_64-unknown-illumos)	58s
cross-tests (ubuntu-latest)	1m 34s
test_wasi (ubuntu-latest, wasm32-wasi)	17s
test_wasm (ubuntu-latest)	21s
	30m 35s

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