doc: add linux/riscv64 valid combination

Mention valid combination GOOS=linux and GOARCH=riscv64
in the "Installing Go from source" document.

Updates #27532

Change-Id: I8c1406087132f5c82a2eee5dbcda95d53c64d263
GitHub-Last-Rev: ee74ff9
GitHub-Pull-Request: #41063
Reviewed-on: https://go-review.googlesource.com/c/go/+/250997
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>

doc/install-source.html

@@ -600,6 +600,9 @@ <h2 id="environment">Optional environment variables</h2>
 <td></td><td><code>linux</code></td> <td><code>mips64le</code></td>
 </tr>
 <tr>
+<td></td><td><code>linux</code></td> <td><code>riscv64</code></td>
+</tr>
+<tr>
 <td></td><td><code>linux</code></td> <td><code>s390x</code></td>
 </tr>
 <tr>