brew安装yarn失败。换npm安装。

brew install yarn

错误信息如下：

$ brew install yarn
Error: homebrew-core is a shallow clone. To `brew update` first run:
git -C "/usr/local/Homebrew/Library/Taps/homebrew/homebrew-core" fetch --unshallow
This restriction has been made on GitHub's request because updating shallow
clones is an extremely expensive operation due to the tree layout and traffic of
Homebrew/homebrew-core. We don't do this for you automatically to avoid
repeatedly performing an expensive unshallow operation in CI systems (which
should instead be fixed to not use shallow clones). Sorry for the inconvenience!
==> Downloading https://yarnpkg.com/downloads/1.17.3/yarn-v1.17.3.tar.g
######################################################################## 100.0%
curl: (60) SSL certificate problem: certificate has expired
More details here: https://curl.haxx.se/docs/sslcerts.html

curl performs SSL certificate verification by default, using a "bundle"
of Certificate Authority (CA) public keys (CA certs). If the default
bundle file isn't adequate, you can specify an alternate file
using the --cacert option.
If this HTTPS server uses a certificate signed by a CA represented in
the bundle, the certificate verification probably failed due to a
problem with the certificate (it might be expired, or the name might
not match the domain name in the URL).
If you'd like to turn off curl's verification of the certificate, use
the -k (or --insecure) option.
HTTPS-proxy has similar options --proxy-cacert and --proxy-insecure.
Error: Failed to download resource "yarn"
Download failed: https://yarnpkg.com/downloads/1.17.3/yarn-v1.17.3.tar.gz

使用npm安装yarn 

npm install -g yarn