Silence mkdir. (#28184)

This can fail if run in parallel.

package.json

@@ -24,7 +24,7 @@
     "bundlesize": "bundlesize",
     "check-broken-links": "npm-run-all --parallel --race \"http-server -- _gh_pages/\" blc",
     "css": "npm-run-all css-compile css-prefix css-minify css-copy",
-    "css-copy": "cross-env-shell shx mkdir -p site/docs/$npm_package_version_short/dist/ && cross-env-shell shx cp -r dist/css/ site/docs/$npm_package_version_short/dist/",
+    "css-copy": "cross-env-shell shx --silent mkdir -p site/docs/$npm_package_version_short/dist/ && cross-env-shell shx cp -r dist/css/ site/docs/$npm_package_version_short/dist/",
     "css-main": "npm-run-all css-lint css-compile-main css-prefix-main css-minify-main css-copy",
     "css-docs": "npm-run-all css-compile-docs css-prefix-docs css-minify-docs",
     "css-compile": "npm-run-all --parallel css-compile-*",
@@ -41,7 +41,7 @@
     "css-prefix-main": "postcss --config build/postcss.config.js --replace \"dist/css/*.css\" \"!dist/css/*.min.css\"",
     "css-prefix-docs": "postcss --config build/postcss.config.js --replace \"site/docs/**/*.css\" \"site/docs/**/*.css\"",
     "js": "npm-run-all js-compile js-minify js-copy",
-    "js-copy": "cross-env-shell shx mkdir -p site/docs/$npm_package_version_short/dist/ && cross-env-shell shx cp -r dist/js/ site/docs/$npm_package_version_short/dist/",
+    "js-copy": "cross-env-shell shx --silent mkdir -p site/docs/$npm_package_version_short/dist/ && cross-env-shell shx cp -r dist/js/ site/docs/$npm_package_version_short/dist/",
     "js-main": "npm-run-all js-lint js-compile js-minify-main",
     "js-docs": "npm-run-all js-lint-docs js-minify-docs",
     "js-compile": "npm-run-all --parallel js-compile-* --sequential js-copy",