Set channel = nightly in dist profile

Author: jyn514

src/bootstrap/defaults/config.dist.toml

@@ -12,6 +12,9 @@ extended = true
 [llvm]
 download-ci-llvm = false
 [rust]
+# We have several defaults in bootstrap that depend on whether the channel is `dev` (e.g. `omit-git-hash` and `download-ci-llvm`).
+# Make sure they don't get set when installing from source.
+channel = "nightly"
 download-rustc = false
 
 [dist]