Unmarshal Panic on PipelineType mismatch with stages

[ecrupper]

Description

If a repository has a pipeline type set to yaml and attempt to use Go syntax in their pipeline, the compiler will panic during parsing.

This only happens with stages according to my limited testing.

Steps (OK)

version: "1"

steps:
  - name: example
    image: {{ default "alpine:latest" .image }}
    commands:
      - echo "Hello World"

This pipeline will give an appropriate error:

unable to unmarshal yaml: yaml: unmarshal errors:
  line 5: cannot unmarshal !!map into string 

Stages (Not OK)

version: "1"

stages:
  test:
    steps:
      - name: example
        image: {{ default "alpine:latest" .image }}
        commands:
          - echo "Hello World"

Panic:

panic: runtime error: hash of unhashable type yaml.MapSlice [recovered]
	panic: runtime error: hash of unhashable type yaml.MapSlice [recovered]
	panic: runtime error: hash of unhashable type yaml.MapSlice [recovered]
	panic: runtime error: hash of unhashable type yaml.MapSlice

goroutine 1 [running]:
github.com/buildkite/yaml.handleErr(0xc0005ed028)
	/home/runner/go/pkg/mod/github.com/buildkite/[email protected]/yaml.go:249 +0x6d
panic({0x297a400?, 0xc0001fcde0?})
	/opt/hostedtoolcache/go/1.21.8/x64/src/runtime/panic.go:914 +0x21f
github.com/buildkite/yaml.handleErr(0xc0005ecd80)
	/home/runner/go/pkg/mod/github.com/buildkite/[email protected]/yaml.go:249 +0x6d
panic({0x297a400?, 0xc0001fcde0?})
	/opt/hostedtoolcache/go/1.21.8/x64/src/runtime/panic.go:914 +0x21f
github.com/buildkite/yaml.handleErr(0xc0005ec6f0)
	/home/runner/go/pkg/mod/github.com/buildkite/[email protected]/yaml.go:249 +0x6d
panic({0x297a400?, 0xc0001fcde0?})
	/opt/hostedtoolcache/go/1.21.8/x64/src/runtime/panic.go:914 +0x21f
github.com/buildkite/yaml.(*decoder).mappingSlice(0xc0000bd620, 0xc0004a6ee0, {0x28a67a0?, 0xc0005f7ef0?, 0xc0005eb6e8?})
	/home/runner/go/pkg/mod/github.com/buildkite/[email protected]/decode.go:725 +0x54f

.......

This appears to be a potential issue with the buildkite yaml library rather than our code, but given that that library is completely deprecated, we might have to get creative with how we address this.

Value

Avoids server panics

Useful Information

In description

1. What is the output of vela --version?

v0.23.4