Exclude String from TaintSteps

For `os.dirEntry` and `os.unixDirent` which are only available
on unix and Windows respectively.

Author: mbg

go/ql/test/library-tests/semmle/go/frameworks/TaintSteps/TaintStep.expected

@@ -864,7 +864,6 @@
 | os.fileWithoutReadFrom.WriteString | file://:0:0:0:0 | [summary param] 0 in WriteString | file://:0:0:0:0 | [summary] to write: Argument[-1] in WriteString |
 | os.rawConn.Read | file://:0:0:0:0 | [summary param] -1 in Read | file://:0:0:0:0 | [summary] to write: Argument[0] in Read |
 | os.rawConn.Write | file://:0:0:0:0 | [summary param] 0 in Write | file://:0:0:0:0 | [summary] to write: Argument[-1] in Write |
-| os.unixDirent.String | file://:0:0:0:0 | [summary param] -1 in String | file://:0:0:0:0 | [summary] to write: ReturnValue in String |
 | path.Base | file://:0:0:0:0 | [summary param] 0 in Base | file://:0:0:0:0 | [summary] to write: ReturnValue in Base |
 | path.Clean | file://:0:0:0:0 | [summary param] 0 in Clean | file://:0:0:0:0 | [summary] to write: ReturnValue in Clean |
 | path.Dir | file://:0:0:0:0 | [summary param] 0 in Dir | file://:0:0:0:0 | [summary] to write: ReturnValue in Dir |

go/ql/test/library-tests/semmle/go/frameworks/TaintSteps/TaintStep.ql

@@ -30,7 +30,7 @@ where
     pkg.matches("crypto/rand.%") and
     name = "Read"
     or
-    pkg = ["os.dirEntry", "os.unixDirent"] and name = ["Info", "Name"]
+    pkg = ["os.dirEntry", "os.unixDirent"] and name = ["Info", "Name", "String"]
     or
     // Not available on arm64
     pkg = "vendor/golang.org/x/crypto/internal/poly1305.mac" and name = "Write"