Fix bad variable name error during build on WSL

[timmyyuan]

WSL appends Windows environment variables to PATH, which causes /bin/sh to report a bad variable name error:

export PATH=//home/user/.cargo/bin:$PATH; cd zeroidc && cargo build --release
/bin/sh: 1: export: Files/Git/cmd:/mnt/c/Program: bad variable name
make: *** [make-linux.mk:433: zeroidc] Error 2

This fix adds double quotes to avoid this problem. See also https://askubuntu.com/questions/1354999/bad-variable-name-error-on-wsl

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