Move dev env file to repo root

Use the conventional .env / .env.example location at the repo
root instead of cfg/dev.env / cfg/dev.env.example. .env is what
contributors expect, keeps cfg/ a pure generated-config directory,
and shares the same file if we ever add another dev target that
needs the same overrides.

Signed-off-by: Bryan Frimin <bryan@getprobo.com>

.gitignore

@@ -21,4 +21,4 @@ pkg/server/api/*/v1/types/types.go
 pkg/server/api/*/v1/schema.graphql
 cfg/dev_local.yaml
 cfg/dev.yaml
-cfg/dev.env
+.env