Flake: `TestReaderReadFileHTTPSProxySuccess`

## Logs

```
2023/05/03 21:02:51 http: TLS handshake error from 127.0.0.1:60668: tls: first record does not look like a TLS handshake
2023/05/03 21:02:51 http: response.WriteHeader on hijacked connection from github.com/aws/eks-anywhere/pkg/files_test.pipe (reader_test.go:257)
2023/05/03 21:02:51 http: response.Write on hijacked connection from fmt.Fprintln (print.go:305)
--- FAIL: TestReaderReadFileHTTPSProxySuccess (0.01s)
reader_test.go:140:
Expected
<*errors.errorString | 0xc0004803d0>: {
s: "failed reading file from url [[https://example.com:443/testdata/file.yaml](https://example.com/testdata/file.yaml)]: Get \"[https://example.com:443/testdata/file.yaml\](https://example.com/testdata/file.yaml/)": EOF",
}
to be nil
FAIL
github.com/aws/eks-anywhere/pkg/files coverage: 92.6% of statements
FAIL github.com/aws/eks-anywhere/pkg/files 0.055s
```

## Occurences

- https://github.com/aws/eks-anywhere/actions/runs/4876450182/jobs/8699956129?pr=5773

Start with pkg/files/reader_test.go, especially TestReaderReadFileHTTPSProxySuccess and the references at lines 140 and 257, then inspect the linked Actions run for reproduction details. Reproduce the HTTPS-proxy failure and trace the TLS handshake and hijacked-connection logs; done means the test reliably passes without the reported EOF or response-write errors.

Written by the indexing model from the issue text.