add setting of UCVM_SRC_PATH and UCVM_INSTALL_PATH from env if avail #286

Job	Run time
ucvm-build-cvmh-taper (3.7)	27m 35s
ucvm-build-cvms-cvmh (3.7)	27m 57s
ucvm-build-albacore-cvlsu-ivlsu-wfcvm (3.7)	26m 32s
	1h 22m 4s

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