Build

CI: add toolchain matrix #20

Job	Run time
build_feature_electrum (linux)	47s
build_all_features (nightly, linux)	2m 18s
build_feature_electrum (windows)	2m 1s
build_feature_esplora (linux)	59s
build_all_features (nightly, windows)	4m 23s
build_feature_electrum (macos)	1m 1s
build_no_default_features (linux)	46s
build_feature_esplora (windows)	3m 28s
build_all_features (nightly, macos)	3m 17s
build_feature_esplora (macos)	45s
build_no_default_features (windows)	3m 22s
build_all_features (stable, linux)	1m 0s
build_all_features (stable, windows)	3m 8s
build_no_default_features (macos)	48s
build_all_features (stable, macos)	1m 1s
build_all_features (1.76.0, linux)	2m 43s
build_all_features (1.76.0, windows)	5m 6s
build_all_features (1.76.0, macos)	2m 44s
	39m 37s

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