tools: add some options and comments to `shell.nix` #1344

Job	Run time
Build slim tarball	1m 34s
aarch64-darwin: with shared libraries	25m 54s
x86_64-linux: with shared libraries	28m 14s
aarch64-linux: with shared libraries	24m 7s
x86_64-darwin: with shared libraries	36m 59s
	1h 56m 48s

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