test(build): generalize test assertion for non-rustup env

Author: weihanglo

tests/testsuite/build.rs

@@ -261,8 +261,8 @@ error: unexpected argument '-z' found
 
   tip: a similar argument exists: '-Z'
 
-Usage: cargo [+toolchain] [OPTIONS] [COMMAND]
-       cargo [+toolchain] [OPTIONS] -Zscript <MANIFEST_RS> [ARGS]...
+Usage: cargo [..][OPTIONS] [COMMAND]
+       cargo [..][OPTIONS] -Zscript <MANIFEST_RS> [ARGS]...
 
 For more information, try '--help'.
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