Credentials error for tasks in perform-release.yml

[sjohnr]

The env variable TOKEN is missing from the environment for tasks in the next-development-version job of perform-release.yml, causing a 401 with "Bad credentials" error from GitHub. For example:

Run ./gradlew createSaganRelease -PnextVersion=$VERSION -PgitHubAccessToken=$TOKEN
> Task :buildSrc:compileJava UP-TO-DATE
> Task :buildSrc:compileGroovy UP-TO-DATE
> Task :buildSrc:pluginDescriptors UP-TO-DATE
> Task :buildSrc:processResources UP-TO-DATE
> Task :buildSrc:classes UP-TO-DATE
> Task :buildSrc:jar UP-TO-DATE
> Task :buildSrc:assemble UP-TO-DATE
> Task :buildSrc:pluginUnderTestMetadata UP-TO-DATE
> Task :buildSrc:compileTestJava NO-SOURCE
> Task :buildSrc:compileTestGroovy NO-SOURCE
> Task :buildSrc:processTestResources NO-SOURCE
> Task :buildSrc:testClasses UP-TO-DATE
> Task :buildSrc:test NO-SOURCE
> Task :buildSrc:validatePlugins UP-TO-DATE
> Task :buildSrc:check UP-TO-DATE
> Task :buildSrc:build UP-TO-DATE
> Task :createSaganRelease FAILED

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':createSaganRelease'.
> 401[{"message":"Bad credentials","documentation_url":"https://docs.github.com/rest"}]