Make tar invocation portable, fix OpenBSD build/test

At least on OpenBSD, tar(1) reads from /dev/rst0 not stdin by default
options must specififed consistently with or without dashes, not mixed.

Specify standard input explicitly to not rely on implementation details.
Use either option style consistently.

src/libexpr/primops/fetchGit.cc

@@ -174,7 +174,7 @@ GitInfo exportGit(ref<Store> store, const std::string & uri,
     Path tmpDir = createTempDir();
     AutoDelete delTmpDir(tmpDir, true);
 
-    runProgram("tar", true, { "x", "-C", tmpDir }, tar);
+    runProgram("tar", true, { "-x", "-f", "-", "-C", tmpDir }, tar);
 
     gitInfo.storePath = store->addToStore(name, tmpDir);
 

tests/tarball.sh

@@ -11,7 +11,7 @@ cp dependencies.nix $tarroot/default.nix
 cp config.nix dependencies.builder*.sh $tarroot/
 
 tarball=$TEST_ROOT/tarball.tar.xz
-(cd $TEST_ROOT && tar c tarball) | xz > $tarball
+(cd $TEST_ROOT && tar cf - tarball) | xz > $tarball
 
 nix-env -f file://$tarball -qa --out-path | grep -q dependencies