Update ReadFromHCLBuildBlock to use the hcp_packer_registry.Description

[nywilken]

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

[lbajolet-hashicorp]

Just left a naming suggestion regarding the build block, but aside from that the code looks good to me, !

[github-actions]

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