tools: add opensslVersion parameter to nix-shell

[panva]

This adds a new opensslVersion parameter to shell.nix and tools/nix/sharedLibDeps.nix that allows selecting which OpenSSL version to link against.

# Default (OpenSSL 3.5.x, same as before)
nix-shell

# OpenSSL 1.1.x
nix-shell --arg opensslVersion '"1.1"'

# OpenSSL 3.0.x
nix-shell --arg opensslVersion '"3.0"'

# OpenSSL 3.5.x
nix-shell --arg opensslVersion '"3.5"'

# OpenSSL 3.6.x
nix-shell --arg opensslVersion '"3.6"'

# Bundled OpenSSL (from deps/openssl)
nix-shell --arg opensslVersion 'null'

[aduh95]

I don't think we should add that if we're not testing for it, it will be hard to maintain it. Instead, folks can already pick which version to link against with e.g. nix-shell --arg sharedLibDeps 'import ./tools/nix/sharedLibDeps.nix {} // { openssl = (import ./tools/nix/pkgs.nix {}).openssl_1_1; }' – it's not as convenient for users, but if that's hidden inside a .envrc that's not too bad I think.

[panva]

that's good enough for me, alright.