Improve gem bindir support #960

Job	Run time
build-gem	15s
Use gem to package metanorma macos-13 Ruby 3.2.6 CC clang	6m 27s
Use gem to package fontist macos-13 Ruby 3.2.6 CC clang	4m 32s
Use gem to package metanorma ubuntu-20.04 Ruby 3.2.6 CC clang-12	4m 46s
Use gem to package fontist windows-latest Ruby 3.2.6 CC gcc	17m 18s
Use gem to package metanorma macos-14 Ruby 3.2.6 CC clang	5m 20s
Use gem to package fontist ubuntu-20.04 Ruby 3.2.6 CC clang-12	2m 11s
Use gem to package metanorma alpine:3.17 Ruby 3.2.6 CC gcc	5m 41s
Use gem to package fontist macos-14 Ruby 3.2.6 CC clang	2m 21s
Use gem to package fontist alpine:3.17 Ruby 3.2.6 CC gcc	2m 47s
cross-test on ubuntu-22.04	32s
cross-test on ubuntu-24.04	30s
Release gem	1s
	52m 41s

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