envsetup: avoid adding "::" (the cwd) to the PATH

Fix this error:

    rprichard@cashew:/x/aosp/out/target/product/walleye/system/bin$ ls
    bash: ./ls: cannot execute binary file: Exec format error

Bug: none
Test: lunch a target, "echo $PATH | grep ::"
Test: lunch an arm target, cd $OUT/system/bin, ls (
Change-Id: I2c0d5ece85964dffa14d6ac8df168dd4e733162e