fix: xcaddy run on windows with go 1.19

[ydylla]

Hi @mholt,
you were right #116 is a windows specific problem. I found this more detailed issue golang/go#43724 were the reasons for the change are described.

To recap how we got here, the fundamental problem is that lots of Go programs do things like exec.Command("prog") and expect that prog will come from a system directory listed in the PATH. It is a surprise and in some cases a security problem that on Windows prog will be taken from dot instead when prog.exe (or prog.bat, prog.com, ...) exists.

Because there is a special case in getCaddyOutputFile for windows xcaddy does exactly that, but it really wants to use the binary from the current directory.

Programs that want to require the current directory to be used (ignoring PATH) can change "prog" to "./prog" (that works on both Unix and Windows systems). This change ("prog" to "./prog") is compatible with older versions of os/exec.

For me run and build also works without the windows special case, but #33 explicitly added the .exe ending so I kept it.

Closes #116

[mholt]

Thanks for digging into this one and figuring it out! LGTM.