Process: Use more rawStd{Out,Err} const methods instead of mutable ones

Avoid using readAllStandard{Output,Error} mutable methods if possible.
Use non-mutable methods when we are not connected to
readyReadStandard{Output,Error} signals.

Change-Id: I2e830e571b9eab2177fd856bbe06dfc5137d9c01
Reviewed-by: Orgad Shaneh <orgads@gmail.com>

src/libs/utils/devicefileaccess.cpp

@@ -1035,7 +1035,7 @@ expected_str<QByteArray> UnixDeviceFileAccess::fileContents(const FilePath &file
         return make_unexpected(Tr::tr("Failed reading file \"%1\": %2")
                                    .arg(filePath.toUserOutput(), p.readAllStandardError()));
     }
-    return p.readAllRawStandardOutput();
+    return p.rawStdOut();
 #else
     return make_unexpected(QString("Not implemented"));
 #endif