remove uses of golang.org/x/sys/execabs

[thaJeztah]

the "golang.org/x/sys/execabs" package was introduced to address a security issue on Windows, and changing the default behavior of os/exec was considered a breaking change. go1.19 applied the behavior that was previously implemented in the execabs package;

from the release notes: https://go.dev/doc/go1.19#os-exec-path

Command and LookPath no longer allow results from a PATH search to be found
relative to the current directory. This removes a common source of security
problems but may also break existing programs that depend on using, say,
exec.Command("prog") to run a binary named prog (or, on Windows, prog.exe)
in the current directory. See the os/exec package documentation for information
about how best to update such programs.

On Windows, Command and LookPath now respect the NoDefaultCurrentDirectoryInExePath
environment variable, making it possible to disable the default implicit search
of “.” in PATH lookups on Windows systems.

With those changes, we no longer need to use the execabs package, and we can switch back to os/exec.

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