Add diffutils needed for "terraform fmt --check --diff".

ansible-lint no longer depends on ansible[1][2] which causes it to fail
with: syntax-check: couldn't resolve module/action 'pacman', as the
modules is part of ansible and not ansible-base.

[1] https://bugs.archlinux.org/task/69920
[2] https://github.com/archlinux/svntogit-community/commit/a3ccb42b8dc104a4616a64fac4a1f6dd44ffedca

yaml: truthy value should be one of [false, true] (truthy)
yaml: wrong indentation: expected 4 but found 2 (indentation)
yaml: too few spaces before comment (comments)
yaml: missing starting space in comment (comments)
yaml: too many blank lines (1 > 0) (empty-lines)
yaml: too many spaces after colon (colons)
yaml: comment not indented like content (comments-indentation)
yaml: no new line character at the end of file (new-line-at-end-of-file)
load-failure: Failed to load or parse file
parser-error: couldn't resolve module/action 'hosts'. This often indicates a misspelling, missing collection, or incorrect module path.

With a dynamic inventory and vault_password_file set it will try to use
them, which of course doesn't work in the CI.

https://github.com/ansible-community/ansible-lint/issues/953

Added --diff to the terraform fmt --check command, because it then will show the
offending lines on the pipeline output.