fix(go): Compilation errors on GOOS=windows

[j-dubb-dev]

Issue #, if available:

#1722

Description of changes:

Fixes #1722 by moving the implementation of GetProcessCpuTimeMillis to an os specific extern_{os}.go file. The file naming and use of compiler directives are required for go programs that have dependencies on packages that do no provide the same types/function signatures, etc across build targets. The syscall package is the main culprit here, which does not implement the same functions or types across unix-like and windows versions.

Changed

• Moved current implementation of GetProcessCpuTimeMillis, that only works on unix-like build targets (unix,linux,darwin,freebsd,etc) out of StandardLibrary/runtimes/go/ImplementationFromDafny-go/Time_/externs.go to a file called externs_unix.go in the same directory. This is because the current implementation uses the syscall.Getrusage function and the syscall.Rusage type which are only implemented in the unix-like versions of the syscall package. Adding the compiler directive //go:build !windows along with the file name ensures this implementation will only be used on unix-like build targets and preserves backwards compatibility.

Added

• extern_windows.go added with an implementation of GetProcessCpuTimeMillis that uses the .
  This file utilizes the //go:build windows compiler directive so only windows builds (GOOS=windows or -tags=windows) use this implementation. It uses the GetProcessTimes function from the windows API to acquire the same information as GetRusage
• windows-latest as an os to test with in /library/.github/workflows/library_go_tests.yml

Squash/merge commit message, if applicable:

fix(go): Compilation errors on GOOS=windows

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