t/testenv.sh: fix macOS test temp directory name #669

Job	Run time
Build Linux packages	48m 58s
Build with earliest Git (ubuntu-latest)	6m 49s
Build Cross Linux packages (arm64, debian_12)	20m 55s
Build with earliest Git (macos-latest)	6m 51s
Build with specific Go (1.21.x)	41s
Build with default Git (ubuntu-latest, 1.22.x)	9m 41s
Build with latest Git (ubuntu-latest)	7m 28s
Build with default Git (macos-latest, 1.22.x)	10m 11s
Build on Windows	53m 34s
Build with latest Git (macos-latest)	8m 10s
	2h 53m 18s

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