[Internal] Minor just improvements

- [x] Fixed `runner/just` so it doesn't require `DSTACK_SHIM_UPLOAD_VERSION` and `DSTACK_SHIM_UPLOAD_S3_BUCKET`
- [x] Added `install-frontend`, `run-frontend` and `run-frontend-sky`

Author: peterschmidt85

frontend/.justfile

@@ -12,6 +12,18 @@ install-frontend:
     cd {{source_directory()}}
     npm install
 
+run-frontend:
+    #!/usr/bin/env bash
+    set -e
+    cd {{source_directory()}}
+    npm run start
+
+run-frontend-sky:
+    #!/usr/bin/env bash
+    set -e
+    cd {{source_directory()}}
+    npm run start-sky
+
 build-frontend:
     #!/usr/bin/env bash
     set -e

runner/.justfile

@@ -26,10 +26,10 @@ default:
     @just --list
 
 # Version of the runner and shim to upload
-export version := env_var("DSTACK_SHIM_UPLOAD_VERSION")
+export version := env("DSTACK_SHIM_UPLOAD_VERSION", "0.0.0")
 
 # S3 bucket to upload binaries to
-export s3_bucket := env_var("DSTACK_SHIM_UPLOAD_S3_BUCKET")
+export s3_bucket := env("DSTACK_SHIM_UPLOAD_S3_BUCKET", "dstack-runner-downloads-stgn")
 
 # Download URLs
 export runner_download_url := "s3://" + s3_bucket + "/" + version + "/binaries/dstack-runner-linux-amd64"