Update the instruction on mirror settings

[nevill]

According to the source, using npm_config_sharp_libvips_binary_host is enough to install the binary from mirror host.
In fact, it doesn't work if sharp_binary_host is set to "https://npm.taobao.org/mirrors/sharp" (should be "https://npm.taobao.org/mirrors/sharp-libvips/v8.10.0/").

[coveralls]

Coverage remained the same at 100.0% when pulling 8e1f2d6 on nevill:develop into 68ccba8 on lovell:master.

[lovell]

Hi, there are potentially two binaries downloaded at install time, one for sharp and one for libvips.

https://sharp.pixelplumbing.com/install#chinese-mirror

Alibaba provide a mirror site based in China containing binaries for both sharp and libvips.

The change in this PR removes use of the mirror for the sharp binary. Is this intentional?

[lovell]

I've just tested the documented approach and it works for me. Please ensure you're using the latest version.

[nevill]

Oh well, I see.
I was thinking the problem here is the false document.

Because config variable npm_config_sharp_binary_host is not used at all according to the source (the latest).

const distBaseUrl = process.env.npm_config_sharp_dist_base_url || process.env.SHARP_DIST_BASE_URL || `${distHost}/v${minimumLibvipsVersionLabelled}/`;

Hi, there are potentially two binaries downloaded at install time, one for sharp and one for libvips.

Then, I realized that another binary is downloaded via package prebuild-install. So, npm_config_sharp_binary_host is considered. 😃