Include specific npm proxy config, to ensure it works reliably

On my machine, node 10 + npm 6.4.1 seems to work fine, but using
some older node or npm version combinations skips the proxy
entirely, or sometimes has issues due to Request sending bad
CONNECT requests in concert with global-agent. This fixes that,
by ensuring npm 100% understands what's going on.

Author: pimterry

src/interceptors/fresh-terminal.ts

@@ -255,6 +255,9 @@ export class TerminalInterceptor implements Interceptor {
                 'GLOBAL_AGENT_HTTP_PROXY': `http://localhost:${proxyPort}`,
                 // Used by some CGI engines to avoid 'httpoxy' vulnerability
                 'CGI_HTTP_PROXY': `http://localhost:${proxyPort}`,
+                // Used by npm, for versions that don't support HTTP_PROXY etc
+                'npm_config_proxy': `http://localhost:${proxyPort}`,
+                'npm_config_https_proxy': `http://localhost:${proxyPort}`,
 
                 // Trust cert when using OpenSSL with default settings
                 'SSL_CERT_FILE': this.config.https.certPath,