Cannot find `abc` binary when running `heir-opt` as a standalone binary

## Issue

When running heir-opt as a standalone binary, it cannot find the abc binary.

## To reproduce

(This assumes we are in the `heir` directory)

1. Build `heir-opt` using bazel
- I use `bazel build @heir//tools:heir-opt`
- There should be a folder under `heir/bazel-bin/tools/heir-opt`. The abc binary should also be located at `heir/bazel-bin/external/edu_berkeley_abc/abc`

2. Run heir-opt from the bazel-bin directory with a --yosys-optimizer
- I ran `./bazel-bin/tools/heir-opt --yosys-optimizer --canonicalize --cse heir/tests/yosys_optimizer/add_one.mlir`
- There should be no visible output, whereas we expect for it to show the modified mlir output.

3. Run heir-opt again, but this time use `strace`
- I ran ` strace ./bazel-bin/tools/heir-opt --yosys-optimizer --canonicalize --cse heir/tests/yosys_optimizer/add_one.mlir`
- The output is quite large so I've pasted it [here](https://gist.github.com/kevaundray/8ca04c8ae92e0c99e7a9de8e67f28c46)
- The line `read(3, "sh: 1: external/edu_berkeley_abc"..., 4096) = 48` seems to indicate that it cannot find the abc tool

## Solution

What fixed it for me was telling bazel to link to the output location of wherever the abc binary was once it was built.

The modification was made in `heir/tools/BUILD`.

I changed `"HEIR_ABC_BINARY=\\\"$(rootpath @edu_berkeley_abc//:abc)\\\"",` to `"HEIR_ABC_BINARY=\\\"$(location @edu_berkeley_abc//:abc)\\\"",`

## Observations

- Everything works for me when running from bazel, ie bazel build and bazel run work fine.
- After the change, bazel build and bazel run still work fine for me.
- The location seems to be an absolute path, so as long as the abc binary is not moved, it works. Otherwise, I had to override the HEIR_ABC_BINARY environment variable to point to the new location of the abc binary