Use POSIX-compliant . instead of source in .profile

The invocation to `source` introduced in 850adef is not POSIX-compliant,
and not available in some shells. Use `.` instead, which will work in
all shells. Concretely, this was failing in Docker builds, where
`/bin/sh` is not Bash.

src/cli/self_update/shell.rs

@@ -98,7 +98,7 @@ pub trait UnixShell {
     }
 
     fn source_string(&self) -> Result<String> {
-        Ok(format!(r#"source "{}/env""#, cargo_home_str()?))
+        Ok(format!(r#". "{}/env""#, cargo_home_str()?))
     }
 }