Only show advice to use cargo update --precise for non-local packages

Packages in the local workspace can't get updated this way; the user
just needs to point to a different source, or otherwise update the
package themselves.

src/cargo/ops/cargo_compile.rs

@@ -667,7 +667,7 @@ pub fn create_bcx<'a, 'cfg>(
                 } else {
                     String::new()
                 }
-            } else {
+            } else if !unit.is_local() {
                 format!(
                     "Either upgrade to rustc {} or newer, or use\n\
                      cargo update -p {}@{} --precise ver\n\
@@ -678,6 +678,8 @@ pub fn create_bcx<'a, 'cfg>(
                     unit.pkg.name(),
                     current_version,
                 )
+            } else {
+                String::new()
             };
 
             anyhow::bail!(

tests/testsuite/rust_version.rs

@@ -124,10 +124,7 @@ fn rust_version_too_high() {
         .with_status(101)
         .with_stderr(
             "error: package `foo v0.0.1 ([..])` cannot be built because it requires \
-             rustc 1.9876.0 or newer, while the currently active rustc version is [..]\n\
-             Either upgrade to rustc 1.9876.0 or newer, or use\n\
-             cargo update -p foo@0.0.1 --precise ver\n\
-             where `ver` is the latest version of `foo` supporting rustc [..]",
+             rustc 1.9876.0 or newer, while the currently active rustc version is [..]\n\n",
         )
         .run();
     p.cargo("build --ignore-rust-version").run();