feat(main): include ".exe" when building for Windows

When the platform OS is Windows, the output file name should be appended
with ".exe" to match behavior with 'go build', and to allow the
executable to be ran.

main.go

@@ -109,7 +109,8 @@ Bakelite compiles the packages named by the import paths for multiple GOOS and
 GOARCH combinations. It does not install their results.
 
 When compiling a package, Bakelite writes the result to output files named
-after the source directory in the form "$package_$goos_$goarch".
+after the source directory in the form "$package_$goos_$goarch". The '.exe'
+suffix is added when writing a Windows executable.
 
 Multiple packages may be given to Bakelite, the result of each are saved as
 described in the preceding paragraph.
@@ -173,6 +174,10 @@ See also: go build, go install, go clean.
 func build(ctx context.Context, platform Platform, pkg string) error {
 	name := fmt.Sprintf("%s-%s-%s", filepath.Base(pkg), platform.OS, platform.Arch)
 
+	if platform.OS == OS_WINDOWS {
+		name += ".exe"
+	}
+
 	env := kvs{
 		"GOOS":   string(platform.OS),
 		"GOARCH": string(platform.Arch),