[bug] global environment int variables cause golang error

If global environment variables are set in the top level pipeline file as an int, it'll cause a golang error and won't be processed

```yaml
env:
myenv: 1

steps:
- label: build image
agents:
queue: kubernetes
plugins:
- kubernetes:
podSpec:
containers:
- image: alpine:latest
command: [echo]
env:
- name: DOCKER_HOST
value: tcp://localhost:2375
args:
- "Hello, world!"
```

Also, the env variables should be merged, so any global envs should be merged into the local step envs