x/playground: cgo builds fail (e.g., importing "net" package) because "gcc" executable not found in $PATH

The playground is currently broken. e.g. https://play.golang.org/p/NbF8QOGEaYB

```go
package main

import (
	"fmt"
	"net"
)

func main() {
	fmt.Println(net.ParseIP("1.2.3.4"))
}
```

Results in:

```
# net
cgo: exec gcc: exec: "gcc": executable file not found in $PATH

Go build failed.
```

/cc @dmitshur 