Bazelisk does not honor locally installed bazel version

On our CI docker we install Bazel version from the official Debian image,
that corresponds to the `.bazelversion` file, by using official Bazel version:

https://releases.bazel.build/1.0.0/rc2/bazel_1.0.0rc2-linux-x86_64.deb

In the same time we install bazelisk, version 1.0 on CI docker image.

If I run `bazelisk build :release` and .bazelversion is 1.0.0rc2, and locally the right version of Bazel is installed, I would expect that Bazelisk is smart enough, to use that version.

What happens, is yet another instance of Bazel is downloaded by Bazelisk:

```
davido@wizball:~/projects/gerrit$ bazel version
Build label: 1.0.0rc2
Build target: bazel-out/k8-opt/bin/src/main/java/com/google/devtools/build/lib/bazel/BazelServer_deploy.jar
Build time: Fri Sep 6 12:17:09 2019 (1567772229)
Build timestamp: 1567772229
Build timestamp as int: 1567772229

davido@wizball:~/projects/gerrit$ which bazel
/usr/bin/bazel

davido@wizball:~/projects/gerrit$ cat .bazelversion
1.0.0rc2

davido@wizball:~/projects/gerrit$ bazelisk build java/com/google/gerrit/server:server
2019/09/10 21:43:16 Downloading https://releases.bazel.build/1.0.0/rc2/bazel-1.0.0rc2-linux-x86_64...
INFO: Writing tracer profile to '/home/jenkins/.cache/bazel/_bazel_jenkins/4e8644684552b40c50dc624b79e09982/command.profile.gz'
INFO: Invocation ID: 3e92d1fa-39a5-4266-a806-642073c70b89
INFO: Analyzed target //java/com/google/gerrit/server:server (0 packages loaded, 0 targets configured).
INFO: Found 1 target...
Target //java/com/google/gerrit/server:server up-to-date:
bazel-bin/java/com/google/gerrit/server/libserver.jar
INFO: Elapsed time: 0.625s, Critical Path: 0.21s
INFO: 0 processes.
INFO: Build completed successfully, 1 total action
```

And now yet additional Bazel version was downloaded by Bazelisk and is located here:

```
/home/davido/.cache/bazelisk/bin/bazelbuild/bazel-1.0.0rc2-linux-x86_64
```

Start by reproducing `bazelisk build :release` with `.bazelversion` set to `1.0.0rc2`, while `/usr/bin/bazel` reports that version. Inspect Bazelisk's launcher behavior and the downloaded path under `~/.cache/bazelisk/bin/bazelbuild/`. Done means the locally installed matching Bazel is honored without downloading another instance.

Written by the indexing model from the issue text.