Fix cross-compilation errors when CGO_CFLAGS/CGO_LDFLAGS is set (#30749)

When you cross-compile Gitea and you specify one of the envrionment
variables related to C flags, cgo will fail to build the generator
programs (e.g. generate-bindata) because GOOS and GOARCH are unset, but
those additional flags variables are not unset together with those.

To solve this issue, the simplest way that I've found is to disable cgo
in the `go generate` command as it's not really used there.

For example, I've had this problem with cross-compiling Gitea on FreeBSD
x86_64 to ARMv7 where it's necessary to pass `--target` to `clang` via
`CGO_CFLAGS`:
```
GOOS=freebsd \
GOARCH=arm \
GGOARM=7 \
CGO_ENABLED=1 \
SYSROOT=/usr/local/freebsd-sysroot/armv7 \
CC=clang \
CGO_CFLAGS="--target=armv7-unknown-freebsd13.2-gnueabihf" \
TAGS="bindata sqlite sqlite_unlock_notify" \
make SHELL='sh -x' build
```

```
Running go generate...
# runtime/cgo
In file included from gcc_freebsd_amd64.c:9:
In file included from /usr/include/signal.h:42:
/usr/include/sys/_ucontext.h:44:2: error: unknown type name 'mcontext_t'
modules/migration/schemas_bindata.go:8: running "go": exit status 1
# runtime/cgo
In file included from gcc_freebsd_amd64.c:9:
In file included from /usr/include/signal.h:42:
/usr/include/sys/_ucontext.h:44:2: error: unknown type name 'mcontext_t'
modules/options/options_bindata.go:8: running "go": exit status 1
# runtime/cgo
In file included from gcc_freebsd_amd64.c:9:
In file included from /usr/include/signal.h:42:
/usr/include/sys/_ucontext.h:44:2: error: unknown type name 'mcontext_t'
modules/public/public_bindata.go:8: running "go": exit status 1
# runtime/cgo
In file included from gcc_freebsd_amd64.c:9:
In file included from /usr/include/signal.h:42:
/usr/include/sys/_ucontext.h:44:2: error: unknown type name 'mcontext_t'
modules/templates/templates_bindata.go:8: running "go": exit status 1
gmake[1]: *** [Makefile:781: generate-go] Error 1
*** Error code 2

Stop.
```

But with this fix Gitea compiles successfully.

Makefile

@@ -778,7 +778,7 @@ generate-backend: $(TAGS_PREREQ) generate-go
 .PHONY: generate-go
 generate-go: $(TAGS_PREREQ)
 	@echo "Running go generate..."
-	@CC= GOOS= GOARCH= $(GO) generate -tags '$(TAGS)' ./...
+	@CC= GOOS= GOARCH= CGO_ENABLED=0 $(GO) generate -tags '$(TAGS)' ./...
 
 .PHONY: security-check
 security-check: