Fixes to reversal of environment updates #2763

Job	Run time
Analyse	4s
Hygiene	17s
Cygwin	7s
Build-macOS (4.14.1)	9m 30s
Build-Linux (4.08.1)	1m 54s
Build-Linux (4.09.1)	2m 0s
Test-macOS (4.14.1)	35m 7s
Build-Linux (4.10.2)	1m 45s
Build-Linux (4.11.2)	1m 47s
Build-Linux (4.12.1)	2m 0s
Build-Linux (4.13.1)	1m 53s
Build-Linux (5.0.0)	1m 54s
Build-Linux (5.1.1)	1m 43s
Build-Linux (4.14.1)	1m 46s
Build-Windows (x86_64-pc-cygwin, x86_64-pc-cygwin, 4.14.1)	38m 33s
Build-Windows (i686-w64-mingw32, x86_64-pc-cygwin, 4.14.1)	48m 37s
Build-Windows (x86_64-w64-mingw32, x86_64-pc-cygwin, 4.14.1)	46m 11s
Build-Windows (i686-pc-windows, x86_64-pc-cygwin, 4.14.1)	52m 53s
Build-Windows (x86_64-pc-windows, x86_64-pc-cygwin, 4.14.1)	49m 52s
Solvers-Linux (z3, 4.14.1)	48s
Upgrade-Linux (4.14.1)	1m 14s
Solvers-Linux (0install, 4.14.1)	1m 27s
Cold-Linux	5m 10s
Test-Linux (4.14.1)	12m 5s
Upgrade-macOS (4.14.1)	5m 8s
Solvers-macOS (z3, 4.14.1)	2m 59s
Solvers-macOS (0install, 4.14.1)	2m 37s
	5h 29m 21s

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