Support Podman style of accessing the host network in Cypress tests (#11421)

* Supply '--network slirp4netns:allow_host_loopback=true' argument when launching via Podman

This means that the host.containers.internal address will work as a way
to access the host machine's network from within the container in
Podman. This is eqivalent to '--add-host
host.docker.internal:host-gateway' in Docker.

* Log the locations of generated files for Cypress tests

* Use 'host.containers.internal' to access the host network when using Podman

* Support Podman in email Cypress tests too

* Restrict code that decides between Docker and Podman to run in Cypress plugins

Because it can't run in the browser - it needs to run a command line
command to find out.

* Move logic for HOST_DOCKER_INTERNAL into cfgDirFromTemplate

cypress/e2e/register/email.spec.ts

@@ -29,7 +29,7 @@ describe("Email Registration", () => {
             cy.startHomeserver({
                 template: "email",
                 variables: {
-                    SMTP_HOST: "host.docker.internal",
+                    SMTP_HOST: "{{HOST_DOCKER_INTERNAL}}", // This will get replaced in synapseStart
                     SMTP_PORT: _mailhog.instance.smtpPort,
                 },
             }).then((_homeserver) => {