Bazelisk corrupts `PATH` if it contains `=`

I've just found out that if `PATH` contains `=` (which in my case was due to some corruption in my `~/.profile`), `bazelisk` will cause `bazel` to run with an almost empty `PATH` (it will only contain the `bazelisk` cache path), causing `bazel` to fail in horrible ways.

I've double-checked that calling the `bazel` binary directly will have the `PATH` variable correctly passed around, while having it called wrapped by bazelisk is broken. To reproduce, just
```bash
export PATH=$PATH:/=
```

Start by reproducing the reported failure with `export PATH=$PATH:/=` and compare Bazelisk-wrapped execution with calling the Bazel binary directly. Trace Bazelisk's PATH handling; done when a PATH containing `=` is passed to Bazel intact rather than reduced to the cache path.

Written by the indexing model from the issue text.