docker: fix the root cause instead of the proximate cause

Author: jyn514

src/bin/cratesfyi.rs

@@ -182,7 +182,7 @@ pub fn main() {
             }
 
             let mut builder = RustwideBuilder::init().unwrap();
-            builder.add_essential_files().expect("failed to add essential files");
+            builder.update_toolchain().expect("failed to add essential files");
         } else if let Some(_) = matches.subcommand_matches("lock") {
             docbuilder.lock().expect("Failed to lock");
         } else if let Some(_) = matches.subcommand_matches("unlock") {

src/docbuilder/rustwide_builder.rs

@@ -93,7 +93,7 @@ impl RustwideBuilder {
         })
     }
 
-    fn update_toolchain(&mut self) -> Result<()> {
+    pub fn update_toolchain(&mut self) -> Result<()> {
         // Ignore errors if detection fails.
         let old_version = self.detect_rustc_version().ok();
 
@@ -112,7 +112,6 @@ impl RustwideBuilder {
 
     fn detect_rustc_version(&self) -> Result<String> {
         info!("detecting rustc's version...");
-        self.toolchain.install(&self.workspace)?;
         let res = Command::new(&self.workspace, self.toolchain.rustc())
             .args(&["--version"])
             .log_output(false)
@@ -128,7 +127,7 @@ impl RustwideBuilder {
         }
     }
 
-    pub fn add_essential_files(&mut self) -> Result<()> {
+    fn add_essential_files(&mut self) -> Result<()> {
         self.rustc_version = self.detect_rustc_version()?;
         let rustc_version = parse_rustc_version(&self.rustc_version)?;