Update download URL to always use "latest" release

Minor adjustment to README.md; removes the need to update the URL whenever a new release is made.

Currently the instructions read:
```sh
curl -L https://github.com/a8m/envsubst/releases/download/v1.2.0/envsubst-`uname -s`-`uname -m` -o envsubst
chmod +x envsubst
sudo mv envsubst /usr/local/bin
```
which hard-codes the release version number in the URL - currently the "latest" release is 1.4.2, not 1.2.0.

Github has a "latest release" URL feature, so the URL can be rewritten as:
```sh
curl -sL https://github.com/a8m/envsubst/releases/latest/download/envsubst-`uname -s`-`uname -m` -o envsubst
chmod +x envsubst
sudo mv envsubst /usr/local/bin
```
(BTW the curl `-sL` rather than just `-L` will also disable the progress bar; download is so quick progress bar isn't useful/needed.)