PACKER_BUILD_NAME environment var is not populated with expected value

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#### Overview of the Issue

PACKER_BUILD_NAME gets populated with the original source name, despite the source name being overridden.

#### Reproduction Steps

Given the template below, I would expect
PACKER_BUILD_NAME=workshop
However, what I'm actually seeing is
PACKER_BUILD_NAME=source1

Note that the 'only' attribute using the re-named name works as expected. In addition, the packer log output also uses the re-named name. Hence why I would think the env variable would have the same name in it. Apologies if this is working as expected but the behavior seems mismatched to me, and the documentation isn't terribly clear.

The MY_PACKER_BUILD_NAME I'm using as a workaround, for reference.

### Packer version

1.9.1

### Simplified Packer Template

source "amazon-ebs" "source1" {
...
}

build {

source "source.amazon-ebs.source1" {
name = "workshop"
}

provisioner "powershell" {
only = ["amazon-ebs.workshop"]
environment_vars = ["MY_PACKER_BUILD_NAME=${source.name}"]
script = "..."
}

}

### Operating system and Environment details

running packer on debian x64 against windowd 2019 x64 on aws

### Log Fragments and crash.log files

amazon-ebs.workshop: PACKER_BUILD_NAME is source1
amazon-ebs.workshop: MY_PACKER_BUILD_NAME is workshop