Flaky test: TestRuncWorkerExec

[jedevc]

See #3946 (review).

For example, https://github.com/moby/buildkit/actions/runs/5857883091/job/15882282244:

=== Failed
=== FAIL: worker/runc TestRuncWorkerExec (1.99s)
    common.go:77: Stdout: hello
    common.go:78: Stderr: 
    common.go:114: Stdout: 
    common.go:115: Stderr: 
    common.go:116: 
        	Error Trace:	/src/worker/tests/common.go:116
        	Error:      	Received unexpected error:
        	            	exit code: 1
        	            	github.com/moby/buildkit/util/stack.Enable
        	            		/src/util/stack/stack.go:77
        	            	github.com/moby/buildkit/executor/runcexecutor.exitError
        	            		/src/executor/runcexecutor/executor.go:375
        	            	github.com/moby/buildkit/executor/runcexecutor.(*runcExecutor).Run
        	            		/src/executor/runcexecutor/executor.go:336
        	            	github.com/moby/buildkit/worker/tests.TestWorkerExec.func2
        	            		/src/worker/tests/common.go:87
        	            	golang.org/x/sync/errgroup.(*Group).Go.func1
        	            		/src/vendor/golang.org/x/sync/errgroup/errgroup.go:75
        	            	runtime.goexit
        	            		/usr/local/go/src/runtime/asm_amd64.s:1598
        	            	container keiv4biu1347vzpqkjjunbkmu has exited with error
        	            	github.com/moby/buildkit/executor/runcexecutor.(*runcExecutor).Exec
        	            		/src/executor/runcexecutor/executor.go:410
        	            	github.com/moby/buildkit/worker/tests.TestWorkerExec
        	            		/src/worker/tests/common.go:107
        	            	github.com/moby/buildkit/worker/runc.TestRuncWorkerExec
        	            		/src/worker/runc/runc_test.go:218
        	            	testing.tRunner
        	            		/usr/local/go/src/testing/testing.go:1576
        	            	runtime.goexit
        	            		/usr/local/go/src/runtime/asm_amd64.s:1598
        	Test:       	TestRuncWorkerExec

[crazy-max]

Repro with dumped context in https://github.com/moby/buildkit/actions/runs/5880025563/job/15945495531?pr=4142#step:7:3015

logs.txt