[rubygems/rubygems] Don't upcase Windows ENV when backing it up #498

Job	Run time
cargo test	36s
cargo clippy	25s
make (yjit-bindgen, To fix: use patch in logs, --with-gcc=clang-12 --enable-yjit=dev)	3m 51s
make (check, RUSTC='rustc +1.58.0', 1.58.0)	17m 22s
make (check, --enable-yjit=dev)	25m 42s
make (check, --enable-yjit=dev, --yjit-call-threshold=1 --yjit-verify-ctx --yjit-code-gc)	28m 42s
make (test-bundled-gems, --enable-yjit=dev)	20m 8s
make (yjit-bench, --enable-yjit=dev, --yjit-stats, true)	5m 25s
YJIT Ubuntu result	1s
	1h 42m 12s

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