Introduce KUBESEAL_VERSION for Linux installation (#1275)

* Introduce KUBESEAL_VERSION for Linux installation

This change introduces a variable `KUBESEAL_VERSION` in the README's
example of how to install the `kubeseal` tools on Linux.
This makes it quicker and easier to just copy-paste the commands in
order to install a certain version.

💡 A good way to inspect this commit is:

```shell
git show --color-words='.'
```

Signed-off-by: Andreas Lindhé <andreas@lindhe.io>

README.md

@@ -379,8 +379,9 @@ nix-env -iA nixpkgs.kubeseal
 The `kubeseal` client can be installed on Linux, using the below commands:
 
 ```bash
-wget https://github.com/bitnami-labs/sealed-secrets/releases/download/<release-tag>/kubeseal-<version>-linux-amd64.tar.gz
-tar -xvzf kubeseal-<version>-linux-amd64.tar.gz kubeseal
+KUBESEAL_VERSION='' # Set this to, for example, KUBESEAL_VERSION='0.23.0'
+wget "https://github.com/bitnami-labs/sealed-secrets/releases/download/v${KUBESEAL_VERSION:?}/kubeseal-${KUBESEAL_VERSION:?}-linux-amd64.tar.gz"
+tar -xvzf kubeseal-${KUBESEAL_VERSION:?}-linux-amd64.tar.gz kubeseal
 sudo install -m 755 kubeseal /usr/local/bin/kubeseal
 ```