Remove .exe suffix when cross-compiling on Windows (#27448)

When compiling GItea for Linux on Windows, you get a `gitea.exe` file as
output, but because it's a Linux executable, the `.exe` extension is
unnecessary.

This PR adds a check for `GOOS` environment variable in addition to
`OS`.

Makefile

@@ -49,10 +49,14 @@ ifeq ($(HAS_GO), yes)
 	CGO_CFLAGS ?= $(shell $(GO) env CGO_CFLAGS) $(CGO_EXTRA_CFLAGS)
 endif
 
-ifeq ($(OS), Windows_NT)
+ifeq ($(GOOS),windows)
-	GOFLAGS := -v -buildmode=exe
+	IS_WINDOWS := yes
-	EXECUTABLE ?= gitea.exe
+else ifeq ($(patsubst Windows%,Windows,$(OS)),Windows)
-else ifeq ($(OS), Windows)
+	ifeq ($(GOOS),)
+		IS_WINDOWS := yes
+	endif
+endif
+ifeq ($(IS_WINDOWS),yes)
 	GOFLAGS := -v -buildmode=exe
 	EXECUTABLE ?= gitea.exe
 else