ls: ensure deterministic output for truncated platforms

[crazy-max]

relates to https://github.com/docker/buildx/actions/runs/11169979364/job/31051857680#step:6:1094

=== FAIL: commands TestTruncPlatforms/all_preferred (0.00s)
    ls_test.go:171: 
        	Error Trace:	/home/runner/work/buildx/buildx/commands/ls_test.go:171
        	Error:      	Not equal: 
        	            	expected: "linux/arm64*, linux/arm* (+3), darwin/arm64*, windows/arm64*"
        	            	actual  : "linux/arm64*, linux/arm* (+3), windows/arm64*, darwin/arm64*"
        	            	
        	            	Diff:
        	            	--- Expected
        	            	+++ Actual
        	            	@@ -1 +1 @@
        	            	-linux/arm64*, linux/arm* (+3), darwin/arm64*, windows/arm64*
        	            	+linux/arm64*, linux/arm* (+3), windows/arm64*, darwin/arm64*
        	Test:       	TestTruncPlatforms/all_preferred
    --- FAIL: TestTruncPlatforms/all_preferred (0.00s)

=== FAIL: commands TestTruncPlatforms (0.00s)