fix: Exempt dependabot from the changelog workflow

[codingllama]

The changelog workflow fails because the -reviewers flag is empty. I think that was always the case, but now that the workflow is required this blocks all dependabot PRs.

Example failure:

Run cd .github/shared-workflows/bot && go run main.go -workflow=changelog -token="***" -reviewers=""

2024/11/01 13:42:03 Failed to parse flags: 
ERROR REPORT:
Original Error: *trace.BadParameterError reviewers required for assign and check
Stack Trace:
	/home/runner/work/teleport/teleport/.github/shared-workflows/bot/main.go:141 main.parseFlags
	/home/runner/work/teleport/teleport/.github/shared-workflows/bot/main.go:37 main.main
	/opt/hostedtoolcache/go/1.23.2/x[6](https://github.com/gravitational/teleport/actions/runs/11627067334/job/32388964174?pr=48261#step:4:7)4/src/runtime/proc.go:272 runtime.main
	/opt/hostedtoolcache/go/1.23.2/x64/src/runtime/asm_amd64.s:1[7](https://github.com/gravitational/teleport/actions/runs/11627067334/job/32388964174?pr=48261#step:4:8)00 runtime.goexit
User Message: reviewers required for assign and check.
exit status 1
Error: Process completed with exit code 1.

Reference on shared-workflows:

• https://github.com/gravitational/shared-workflows/blob/340561d4996e85b686c714df97a53cd40c142838/bot/main.go#L141

[codingllama]

It's debatable whether this is the best solution, but it's likely the faster and I just want to get the bots unblocked.

[codingllama]

Example of an affected PR / failed run:

• Bump the rust group with 9 updates #48266
• https://github.com/gravitational/teleport/actions/runs/11627200717/job/32380130999?pr=48266