Github action secrets for Ethereum endpoints
We have some tests now that require access to Goerli and/or Mainnet Etherereum endpoints. The public endpoints are not that reliable leading to test failures. I propose that tests use an environment variable, if defined, for these endpoints. If the environment variables are not defined, the code can continue to use the built-in constants. When running the tests in CI, those variables could be defined by Github action secrets.

The exact errors we see, when the public endpoints fail, vary a lot. This is one example from yesterday:
```
=== RUN   TestGetETHUSDPrice_mainnet
    pricefeed_test.go:24: 
        	Error Trace:	/home/runner/work/atomic-swap/atomic-swap/pricefeed/pricefeed_test.go:24
        	Error:      	Received unexpected error:
        	            	Internal JSON-RPC error.
        	Test:       	TestGetETHUSDPrice_mainnet
--- FAIL: TestGetETHUSDPrice_mainnet (0.17s)
```