Update cheatsheets

nix3-shell

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+---
+syntax: markdown
+tags: [tldr, common]
+source: https://github.com/tldr-pages/tldr.git
+---
+# nix shell
+
+> Start a shell in which the specified packages are available.
+> See also: `tldr nix-shell`. See `tldr nix3 flake` for information about flakes.
+> More information: <https://nixos.org/manual/nix/stable/command-ref/new-cli/nix3-shell.html>.
+
+- Start an interactive shell with some packages from `nixpkgs`:
+
+`nix shell {{nixpkgs#pkg1 nixpkgs#packageSet.pkg2 ...}}`
+
+- Start a shell providing a package from an older version of `nixpkgs` (21.05):
+
+`nix shell {{nixpkgs/nixos-21.05#pkg}}`
+
+- Start a shell with the "default package" from a flake in the current directory, printing build logs if any builds happen:
+
+`nix shell -L`
+
+- Start a shell with a package from a flake on GitHub:
+
+`nix shell {{github:owner/repo#pkg}}`
+
+- Run a command in a shell with a package:
+
+`nix shell {{nixpkgs#pkg}} -c {{some-cmd --someflag 'Some other arguments'}}`