Update ReadFromHCLBuildBlock to use the hcp_packer_registry.Description

In packer v1.8.5, the bucket's description was not properly set in the
bucket object we use for HCP, therefore all the buckets created by
Packer did not have their description updated.

Before the change
```
--- FAIL: TestReadFromHCLBuildBlock (0.00s)
    --- FAIL: TestReadFromHCLBuildBlock/configure_bucket_using_only_hcp_packer_registry_block (0.00s)
        types.bucket_test.go:380: expected the build to to have contents of hcp_packer_registry block but it does not:   &registry.Bucket{
                Slug:         "hcp_packer_registry-block-test",
            -   Description:  "",
            +   Description:  "description from hcp_packer_registry block",
                Destination:  "",
                BucketLabels: {"org": "test"},
                ... // 5 identical fields
              }
FAIL
FAIL    github.com/hashicorp/packer/internal/hcp/registry       1.072s
FAIL
```

After Change
```
~>  go test ./...
?       github.com/hashicorp/packer/internal/hcp/api    [no test files]
ok      github.com/hashicorp/packer/internal/hcp/env    (cached)
ok      github.com/hashicorp/packer/internal/hcp/registry       1.130s

```

internal/hcp/registry/types.bucket.go

@@ -64,23 +64,23 @@ func (b *Bucket) Validate() error {
 }
 
 // ReadFromHCLBuildBlock reads the information for initialising a Bucket from a HCL2 build block
-func (b *Bucket) ReadFromHCLBuildBlock(hcpBlock *hcl2template.BuildBlock) {
+func (b *Bucket) ReadFromHCLBuildBlock(build *hcl2template.BuildBlock) {
 	if b == nil {
 		return
 	}
-	b.Description = hcpBlock.Description
 
-	hcp := hcpBlock.HCPPackerRegistry
-	if hcp == nil {
+	registryBlock := build.HCPPackerRegistry
+	if registryBlock == nil {
 		return
 	}
 
-	b.BucketLabels = hcp.BucketLabels
-	b.BuildLabels = hcp.BuildLabels
+	b.Description = registryBlock.Description
+	b.BucketLabels = registryBlock.BucketLabels
+	b.BuildLabels = registryBlock.BuildLabels
 	// If there's already a Slug this was set from env variable.
 	// In Packer, env variable overrides config values so we keep it that way for consistency.
-	if b.Slug == "" && hcp.Slug != "" {
-		b.Slug = hcp.Slug
+	if b.Slug == "" && registryBlock.Slug != "" {
+		b.Slug = registryBlock.Slug
 	}
 }
 

internal/hcp/registry/types.bucket_test.go

@@ -6,6 +6,7 @@ import (
 	"testing"
 
 	"github.com/google/go-cmp/cmp"
+	"github.com/hashicorp/packer/hcl2template"
 	"github.com/hashicorp/packer/internal/hcp/api"
 )
 
@@ -333,3 +334,51 @@ func TestBucket_PopulateIteration(t *testing.T) {
 		})
 	}
 }
+
+func TestReadFromHCLBuildBlock(t *testing.T) {
+	tc := []struct {
+		desc           string
+		buildBlock     *hcl2template.BuildBlock
+		expectedBucket *Bucket
+	}{
+		{
+			desc: "configure bucket using only hcp_packer_registry block",
+			buildBlock: &hcl2template.BuildBlock{
+				HCPPackerRegistry: &hcl2template.HCPPackerRegistryBlock{
+					Slug:        "hcp_packer_registry-block-test",
+					Description: "description from hcp_packer_registry block",
+					BucketLabels: map[string]string{
+						"org": "test",
+					},
+					BuildLabels: map[string]string{
+						"version":   "1.7.0",
+						"based_off": "alpine",
+					},
+				},
+			},
+			expectedBucket: &Bucket{
+				Slug:        "hcp_packer_registry-block-test",
+				Description: "description from hcp_packer_registry block",
+				BucketLabels: map[string]string{
+					"org": "test",
+				},
+				BuildLabels: map[string]string{
+					"version":   "1.7.0",
+					"based_off": "alpine",
+				},
+			},
+		},
+	}
+	for _, tt := range tc {
+		tt := tt
+		t.Run(tt.desc, func(t *testing.T) {
+			bucket := &Bucket{}
+			bucket.ReadFromHCLBuildBlock(tt.buildBlock)
+
+			diff := cmp.Diff(bucket, tt.expectedBucket, cmp.AllowUnexported(Bucket{}))
+			if diff != "" {
+				t.Errorf("expected the build to to have contents of hcp_packer_registry block but it does not: %v", diff)
+			}
+		})
+	}
+}