Support default values for env var expansion #25524

Job	Run time
contrib-tests-matrix (receiver-0)	7m 20s
contrib-tests-matrix (receiver-1)	8m 58s
contrib-tests-matrix (receiver-2)	9m 2s
contrib-tests-matrix (receiver-3)	8m 0s
contrib-tests-matrix (processor)	6m 41s
contrib-tests-matrix (exporter-0)	3m 18s
contrib-tests-matrix (exporter-1)	10m 15s
contrib-tests-matrix (extension)	6m 19s
contrib-tests-matrix (connector)	4m 7s
contrib-tests-matrix (internal)	6m 41s
contrib-tests-matrix (pkg)	3m 30s
contrib-tests-matrix (cmd-0)	48s
contrib-tests-matrix (cmd-1)	3m 54s
contrib-tests-matrix (other)	3m 58s
contrib_tests	1s
	1h 22m 52s

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