build: build Go SSA serially [issue 3895]

From
#3915 (comment)

According to Ayke, it slows down the build but seems to reliably fix #3895

Author: dkegel-fastly

builder/build.go

@@ -344,6 +344,10 @@ func Build(pkgName, outpath, tmpdir string, config *compileopts.Config) (BuildRe
 		}
 		packageActionIDJobs[pkg.ImportPath] = packageActionIDJob
 
+		// Build the SSA for the given package.
+		ssaPkg := program.Package(pkg.Pkg)
+		ssaPkg.Build()
+
 		// Now create the job to actually build the package. It will exit early
 		// if the package is already compiled.
 		job := &compileJob{