base tests: switch to nightly toolchain before checking formatting of tests with rustfmt

this errored because rustfmt is not available on the master toolchain

ci/base-tests.sh

@@ -42,9 +42,13 @@ set +x
 
 # some lints are sensitive to formatting, exclude some files
 needs_formatting=false
+# switch to nightly
+rustup default nightly
 for file in `find tests -not -path "tests/ui/methods.rs" -not -path "tests/ui/format.rs" -not -path "tests/ui/formatting.rs" -not -path "tests/ui/empty_line_after_outer_attribute.rs" -not -path "tests/ui/double_parens.rs" -not -path "tests/ui/doc.rs" -not -path "tests/ui/unused_unit.rs" | grep "\.rs$"` ; do
 rustfmt ${file} --check  || echo "${file} needs reformatting!" ; needs_formatting=true
 done
+# switch back to master
+rustup default master
 
 if [ "${needs_reformatting}" = true ] ; then
     echo "Tests need reformatting!"