pkg/emulation: add linux build tag for elf

Otherwise, golangci-lint on freebsd complains:

> pkg/emulation/elf.go:14:6: type elfPlatform is unused (unused)
> type elfPlatform struct {
>      ^
> pkg/emulation/elf.go:28:2: var knownELFPlatformHeaders is unused (unused)
> 	knownELFPlatformHeaders     = make(map[string][][]byte)
> 	^
> pkg/emulation/elf.go:29:2: var knownELFPlatformHeadersOnce is unused (unused)
> 	knownELFPlatformHeadersOnce sync.Once
> 	^
> pkg/emulation/elf.go:33:2: var knownELFPlatforms is unused (unused)
> 	knownELFPlatforms = []elfPlatform{
> 	^
> pkg/emulation/elf.go:157:23: func (*elfPlatform).header is unused (unused)
> func (e *elfPlatform) header() ([][]byte, error) {
>                       ^
> pkg/emulation/elf.go:208:6: func getKnownELFPlatformHeaders is unused (unused)
> func getKnownELFPlatformHeaders() map[string][][]byte {
>      ^

Signed-off-by: Kir Kolyshkin <[email protected]>

Author: kolyshkin

pkg/emulation/elf.go

@@ -1,4 +1,4 @@
-//go:build !remote
+//go:build linux && !remote
 
 package emulation