Syntax error with string interpolation - Unexpected identifier in binding

In this PR: https://github.com/dotnet/fsharp/pull/14383 an additional (highly suspect .ToString() was need to get it to compile.

Without ToString():
With ToString():

For the non-ToString() version the compiler produces this:
````
FSharp.Compiler.Service -> C:\kevinransom\fsharp\artifacts\bin\FSharp.Compiler.Service\Debug\netstandard2.0\FSharp.Compiler.Service.d
ll
C:\kevinransom\fsharp\vsintegration\src\FSharp.VS.FSI\fsiSessionToolWindow.fs(538,9): error FS0588: The block following this 'let' is u
nfinished. Every code block is an expression and must have a result. 'let' cannot be the final code element in a block. Consider giving
this block an explicit result. [C:\kevinransom\fsharp\vsintegration\src\FSharp.VS.FSI\FSharp.VS.FSI.fsproj]
C:\kevinransom\fsharp\vsintegration\src\FSharp.VS.FSI\fsiSessionToolWindow.fs(547,9): error FS0010: Unexpected identifier in binding. E
xpected incomplete structured construct at or before this point or other token. [C:\kevinransom\fsharp\vsintegration\src\FSharp.VS.FSI\
FSharp.VS.FSI.fsproj]
C:\kevinransom\fsharp\vsintegration\src\FSharp.VS.FSI\fsiSessionToolWindow.fs(537,5): error FS3118: Incomplete value or function defini
tion. If this is in an expression, the body of the expression must be indented to the same column as the 'let' keyword. [C:\kevinransom
\fsharp\vsintegration\src\FSharp.VS.FSI\FSharp.VS.FSI.fsproj]
````