Triggering cloud ephemeral builds on main-dry-run leads to unexpected
results which is due to how the eventual pipeline is generated.
Closes DINF-165
### Generated pipeline?
The pipeline gets generated based on _what matches first_. We detect
cloud ephemeral builds if there is an environment variable
`CLOUD_EPHEMERAL=true`. We detect main-dry-runs if the branch prefix is
`main-dry-run`...
Now due to the `main-dry-run` match happening before the cloud ephemeral
match a Cloud Ephemeral build on a main dry run gets detected as _just_
a `main-dry-run` build.
#### Alternatives
Sure we can just move the Cloud Ephemeral match earlier, but it's
difficult to say what else might break. We could also just add the we
force the runtype to always be `CloudEphemeral` if we're on the Cloud
Ephemeral pipeline, but I don't want to make the pipline a special
snowflake and detect the current pipeline just for Cloud Ephemeral.
#### Why deny it the deployment?
Ultimately, `main-dry-run` builds are meant for ... `main` not cloud
ephemeral. People can just switch to their original branch and run `sg
cloud eph deploy` and the branch will be deployed with no issue
## Test plan
Executed locally
```
./sg-test cloud eph build
⚠️ Running sg with a dev build, following flags have different default value unless explictly set: skip-auto-update, disable-analytics
⚠️ Triggering Cloud Ephemeral builds from "main-dry-run" branches are not supported. Try renaming the branch to not have the "main-dry-run" prefix as it complicates the eventual pipeline that gets generated
To rename a branch and launch a cloud ephemeral deployment do:
1. git branch -m "main-dry-run/lol" <my-new-name>
2. git push --set-upstream origin <my-new-name>
3. trigger the build by running sg cloud ephemeral build
FAQ https://www.notion.so/sourcegraph/How-to-deploy-my-branch-on-an-ephemeral-Cloud-instance-dac45846ca2a4e018c802aba37cf6465?pvs=4#20cb92ae27464891a9d03650b4d67cee❌ failed to trigger epehemeral build for branch: main-dry-run branch is not supported
```
## Changelog
* sg: deny deployment of `main-dry-run` branches on Cloud Ephemeral.
8a3e4c3bcc