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()?))
     }
 }
 

tests/cli-paths.rs

@@ -29,7 +29,7 @@ export PATH="$HOME/apple/bin"
     const POSIX_SH: &str = "env";
 
     fn source(dir: impl Display, sh: impl Display) -> String {
-        format!("source \"{dir}/{sh}\"\n", dir = dir, sh = sh)
+        format!(". \"{dir}/{sh}\"\n", dir = dir, sh = sh)
     }
 
     #[test]
@@ -273,7 +273,7 @@ export PATH="$HOME/apple/bin"
             assert!(cmd.output().unwrap().status.success());
 
             let new_profile = fs::read_to_string(&profile).unwrap();
-            let expected = format!("{}source \"$HOME/.cargo/env\"\n", FAKE_RC);
+            let expected = format!("{}. \"$HOME/.cargo/env\"\n", FAKE_RC);
             assert_eq!(new_profile, expected);
 
             let mut cmd = clitools::cmd(config, "rustup", &["self", "uninstall", "-y"]);