Segmentation Fault (SIGSEGV) during Keybase installation via Homebrew on macOS

**Description:**

When installing Keybase using Homebrew on macOS, I encountered a segmentation fault (SIGSEGV) during the installation process, specifically while installing command-line tools. This appears to be caused by a nil pointer dereference in the Go code.

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**Steps to Reproduce:**

1. Run `brew install keybase` on macOS.
2. Observe the panic during the installation process.

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**Expected behavior:**

The installation should complete without errors.

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**Actual behavior:**

The installation fails with a segmentation fault.

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**Environment:**

- **macOS Version:** `Darwin Darwin Kernel Version 24.0.0 arm64`
- **Homebrew Version:** Homebrew 4.4.2
- **Keybase Version:** 6.5.0

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**Error Logs:**

```shell
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0x28 pc=0x1016a4484]

goroutine 1 [running]:
github.com/keybase/client/go/install.installCommandLineForBinPath({0x14000470640, 0x3c}, {0x140001be810, 0x16}, 0x1, {0x12a2203e0, 0x14000405d40})
/Users/test/go/src/github.com/keybase/client/go/install/install_darwin.go:710 +0x74
github.com/keybase/client/go/install.installCommandLine({0x101a182a5?, 0x3?}, {0x0?, 0x14000762cd0?}, 0x1, {0x12a2203e0, 0x14000405d40})
/Users/test/go/src/github.com/keybase/client/go/install/install_darwin.go:673 +0x13c
github.com/keybase/client/go/install.Install({0x10248e418, 0x140003cb608}, {0x0, 0x0}, {0x0, 0x0}, {0x1400046a5f0, 0x5, 0x5}, 0x1, ...)
/Users/test/go/src/github.com/keybase/client/go/install/install_darwin.go:494 +0x138
github.com/keybase/client/go/install.InstallAuto({0x10248e418, 0x140003cb608}, {0x0, 0x0}, {0x0, 0x0}, 0x28fa6ae00, {0x12a2203e0, 0x14000405d40})
/Users/test/go/src/github.com/keybase/client/go/install/install_darwin.go:441 +0x1fc
github.com/keybase/client/go/client.(*cmdInstallAuto).Run(0x1400067e440)
/Users/test/go/src/github.com/keybase/client/go/client/cmd_install_osx.go:361 +0x74
main.mainInner(0x140003cb608, {0x0, 0x0, 0x0})
/Users/test/go/src/github.com/keybase/client/go/keybase/main.go:247 +0xb38
main.main()
/Users/test/go/src/github.com/keybase/client/go/keybase/main.go:70 +0x84
```

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**Additional Context:**

The installation process seems to fail while checking `/usr/local/bin/keybase`. It appears related to the Go code at this point:

```shell
/Users/test/go/src/github.com/keybase/client/go/install/install_darwin.go:710
```

Start by reproducing the failure with `brew install keybase` on macOS, then inspect `go/install/install_darwin.go` around lines 673, 710, and 494 using the provided stack trace. Trace the `/usr/local/bin/keybase` check and identify the nil value involved. Done means the installation completes without the SIGSEGV.

Written by the indexing model from the issue text.