Share read for text resource files (#7230)

Fixes #7229 by ensuring that MSBuild's processing of a `.txt` resource holds only the minimal lock: open for read and allowing other processes to read while it's open.

src/Tasks/GenerateResource.cs

@@ -3876,7 +3876,7 @@ internal sealed class LineNumberStreamReader : StreamReader
             private int _col;
 
             internal LineNumberStreamReader(String fileName, Encoding encoding, bool detectEncoding)
-                : base(File.Open(fileName, FileMode.Open, FileAccess.Read), encoding, detectEncoding)
+                : base(File.Open(fileName, FileMode.Open, FileAccess.Read, FileShare.Read), encoding, detectEncoding)
             {
                 _lineNumber = 1;
                 _col = 0;