Install script is broken: due to invalid parameter substitution syntax

[edouard-lopez]

Describe the bug

• PR test: update install.sh script #4071 introduced a parameter expansion that contains a typo, making the syntax invalid in POSIX shell (missing the - dash after the : column)

-  echo "https://github.com/kubeshop/testkube/releases/download/${version}/testkube_${version:1}_${os}_$arch.tar.gz"
+  echo "https://github.com/kubeshop/testkube/releases/download/${version}/testkube_${version:-1}_${os}_$arch.tar.gz"

• Download URL need both tag and version which differ by a prefix.

❯ sh ./install.sh
…
./install.sh: 46: Bad substitution

To Reproduce

❯ sh
$ foo="123"
$ echo "bar=${foo}"
bar=123
$ echo "bar=${foo:1}"  
sh: 3: Bad substitution

Expected behavior

The parameter subsitution should happen

Version / Cluster

• Which testkube version? 699e66c
• What Kubernetes cluster? irrelevant
• What Kubernetes version? irrelevant

Screenshots
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Additional context
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