github action: mage test failed on MacOS

There are test failures while running `mage test` on MacOS

```
Test: github.com/elastic/go-txfile
Run test for package 'github.com/elastic/go-txfile' (binary: false)
# golang.org/x/sys/unix
Error: ../../../go/pkg/mod/golang.org/x/sys@v0.0.0-2020010214[19](https://github.com/elastic/go-txfile/pull/54/checks#step:4:19)24-c96a22e43c9c/unix/syscall_darwin.1_13.go:25:3: //go:linkname must refer to declared function or variable
Error: ../../../go/pkg/mod/golang.org/x/sys@v0.0.0-[20](https://github.com/elastic/go-txfile/pull/54/checks#step:4:20)200102141924-c96a22e43c9c/unix/zsyscall_darwin_amd64.1_13.go:27:3: //go:linkname must refer to declared function or variable
Error: ../../../go/pkg/mod/golang.org/x/sys@v0.0.0-2020010[21](https://github.com/elastic/go-txfile/pull/54/checks#step:4:21)41924-c96a22e43c9c/unix/zsyscall_darwin_amd64.1_13.go:40:3: //go:linkname must refer to declared function or variable
Error: ../../../go/pkg/mod/golang.org/x/sys@v0.0.0-20200102141924-c96a[22](https://github.com/elastic/go-txfile/pull/54/checks#step:4:22)e43c9c/unix/zsyscall_darwin_amd64.go:28:3: //go:linkname must refer to declared function or variable
Error: ../../../go/pkg/mod/golang.org/x/sys@v0.0.0-20200102141924-c96a22e43c9c/unix/zsyscall_darwin_amd64.go:43:3: //go:linkname must refer to declared function or variable
Error: ../../../go/pkg/mod/golang.org/x/sys@v0.0.0-202001021419[24](https://github.com/elastic/go-txfile/pull/54/checks#step:4:24)-c96a22e43c9c/unix/zsyscall_darwin_amd64.go:59:3: //go:linkname must refer to declared function or variable
Error: ../../../go/pkg/mod/golang.org/x/sys@v0.0.0-20200102141924-c96a22e43c9c/unix/zsyscall_darwin_amd64.go:75:3: //go:linkname must refer to declared function or variable
Error: ../../../go/pkg/mod/golang.org/x/sys@v0.0.0-20200102141924-c96a22e43c9c/unix/zsyscall_darwin_amd64.go:90:3: //go:linkname must refer to declared function or variable
Error: ../../../go/pkg/mod/golang.org/x/sys@v0.0.0-20200102141924-c96a22e43c9c/unix/zsyscall_darwin_amd64.go:105:3: //go:linkname must refer to declared function or variable
Error: ../../../go/pkg/mod/golang.org/x/sys@v0.0.0-20200102141924-c96a22e43c9c/unix/zsyscall_darwin_amd64.go:121:3: //go:linkname must refer to declared function or variable
Error: ../../../go/pkg/mod/golang.org/x/sys@v0.0.0-20200102141924-c96a22e43c9c/unix/zsyscall_darwin_amd64.go:121:3: too many errors
FAIL github.com/elastic/go-txfile [build failed]
Test: github.com/elastic/go-txfile/dev-tools/lib/mage/xbuild
Skipping github.com/elastic/go-txfile/dev-tools/lib/mage/xbuild: No tests found
Test: github.com/elastic/go-txfile/internal/cleanup
```

See https://github.com/elastic/go-txfile/pull/54

Start by reproducing the macOS GitHub Action failure with `mage test`, focusing on the `github.com/elastic/go-txfile` build errors from `golang.org/x/sys/unix`. Review the checks linked from pull request #54 and determine the project change needed for the test run to complete successfully on macOS.

Written by the indexing model from the issue text.