Fix relative paths in Gradle tasks

[bjagg]

Describe the bug
Relative paths using FIle() need to be corrected in Gradle scripts

To Reproduce
Steps to reproduce the behavior:

1. Create a new clone
2. Run tomcatInstall
3. Fail to create tmp unpacking directory

Expected behavior
tomcatInstall should succeed

[ChristianMurphy]

@bjagg what environment did you encounter this issue on? Are there additional steps to reproduce the issue?
CI is running ./gradlew tomcatInstall across a few platforms and doesn't appear to be encountering this issue 🤔
https://github.com/uPortal-Project/uPortal-start/actions/runs/1163949348