[dx] Allow to not specify BUILDER for makefile if PLATFORM specified (#1288)

Signed-off-by: Andrei Kvapil <kvapss@gmail.com>

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## What this PR does


### Release note

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```release-note
[dx] Allow to not specify BUILDER for makefile if PLATFORM specified
```

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-->

## Summary by CodeRabbit

* **Refactor**
* Improved handling of the PLATFORM variable to ensure it is only set
when undefined, providing clearer and more predictable behavior.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

scripts/common-envs.mk

@@ -17,6 +17,8 @@ endif
 
 # Get the name of the selected docker buildx builder
 BUILDER ?= $(shell docker buildx inspect --bootstrap | head -n2 | awk '/^Name:/{print $$NF}')
-# Get platforms supported by the builder
-PLATFORM ?= $(shell docker buildx ls --format=json | jq -r 'select(.Name == "$(BUILDER)") | [.Nodes[].Platforms // []] | flatten | unique | map(select(test("^linux/amd64$$|^linux/arm64$$"))) | join(",")')
 
+# Get platforms supported by the builder (only if PLATFORM is not provided)
+ifeq ($(origin PLATFORM), undefined)
+PLATFORM := $(shell docker buildx ls --format=json | jq -r 'select(.Name == "$(BUILDER)") | [.Nodes[].Platforms // []] | flatten | unique | map(select(test("^linux/amd64$$|^linux/arm64$$"))) | join(",")')
+endif