Issue while updating to Geb 8 Micronaut

## Steps to reproduce:

Clone branch `geb8` of https://github.com/micronaut-projects/micronaut-security

The following test fails

```./gradlew :test-suite-geb:test --tests "io.micronaut.security.session.SessionAuthenticationSpec" with:

SessionAuthenticationSpec > verify session based authentication works FAILED
groovy.lang.ReadOnlyPropertyException: Cannot set readonly property: baseUrl for class: geb.Browser
at app//groovy.lang.GroovyObject.setProperty(GroovyObject.java:61)
at app//io.micronaut.security.session.SessionAuthenticationSpec.getBrowser(SessionAuthenticationSpec.groovy:63)
at app//geb.spock.GebSpec.methodMissing(GebSpec.groovy:1)
at io.micronaut.security.session.SessionAuthenticationSpec.verify session based authentication works(SessionAuthenticationSpec.groovy:78)
```

Browser is retrieved from https://github.com/apache/groovy-geb/blob/master/module/geb-core/src/main/groovy/geb/test/GebTestManager.groovy#L48 via https://github.com/apache/groovy-geb/blob/master/module/geb-core/src/main/groovy/geb/test/GebTestManager.groovy#L48
```

Start by checking out the `geb8` branch of `micronaut-projects/micronaut-security` and running `./gradlew :test-suite-geb:test --tests "io.micronaut.security.session.SessionAuthenticationSpec"`. Read `SessionAuthenticationSpec.groovy` around lines 63 and 78, then inspect Geb's `GebTestManager.groovy` around line 48. Done means the session authentication test runs without the `ReadOnlyPropertyException`.

Written by the indexing model from the issue text.