Use environment secrets for Codecov token #3443

Job	Run time
alternative_targets (ubuntu-latest, wasm32-unknown-emscripten)	20s
check-docs	19s
alternative_targets (ubuntu-latest, aarch64-apple-ios)	7s
cross-targets (x86_64-unknown-illumos)	1m 10s
alternative_targets (ubuntu-latest, aarch64-linux-android)	14s
cross-tests (ubuntu-latest)	1m 44s
features_check (ubuntu-latest)	3m 48s
no_std (ubuntu-latest, thumbv6m-none-eabi)	13s
rust_msrv (ubuntu-latest)	56s
rust_versions (ubuntu-latest, stable)	1m 7s
no_std (ubuntu-latest, x86_64-fortanix-unknown-sgx)	25s
test_wasi (ubuntu-latest, wasm32-wasi)	31s
rust_versions (ubuntu-latest, beta)	1m 2s
test_wasm (ubuntu-latest)	15s
rust_versions (ubuntu-latest, nightly)	51s
timezones (ubuntu-latest, ACST-9:30)	1m 15s
timezones (ubuntu-latest, EST4)	1m 14s
timezones (ubuntu-latest, UTC0)	1m 23s
timezones (ubuntu-latest, Asia/Katmandu)	1m 14s
timezones (macos-latest, ACST-9:30)	41s
timezones (macos-latest, EST4)	47s
timezones (macos-latest, UTC0)	31s
timezones (macos-latest, Asia/Katmandu)	43s
timezones (windows-latest, ACST-9:30)	2m 16s
timezones (windows-latest, EST4)	2m 8s
timezones (windows-latest, UTC0)	1m 22s
timezones (windows-latest, Asia/Katmandu)	2m 39s
	29m 15s

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