specify nginx via environment variable or skip test

[nothingmuch]

in the flake devshell we can set environment variables, and do so for BITCOIND_EXE

similarly, we should specify a full path for the test so that if the environment we're in has an nginx that isn't built with streaming support like we require for the websockets stuff then it won't be used by mistake generating test failures

this should provide a better user experience for nix haters too, as the integration test can just skip gracefully unless they opt into it by specifying the nginx they want to use

followup to #1220 (review)

[zealsham]

Hello @nothingmuch i just finished a huge chunk of the tutorial on https://nix.dev and want to take on this as my first nix based solution.

[nothingmuch]

sounds good.

to be clear what this involves:

1. introducing an env variable like BITCOIND_EXE which is used to specify the nginx binary to run in the tests
2. setting this env variable from flake.nix
3. removing the nginxWithStream package from the devshell itself, as manually running nginx shouldn't be required (same as we don't pollute the devshell environment with bitcoind/bitcoin-cli, but still make them available for tests)