Add bin/interpolate-text-template

Modifies the "Generate summary" step from the conda-base repo CI
workflow¹ into a separate script that can be used to interpolate any
text template with environment variables.

I will be using this in the reusable run-build workflow to create the
GitHub Action summary for AWS Batch builds. I also see this being useful
for creating Slack messages for automated builds such as the onstart
and onerror notifications that we use in ncov² if we decide to move the
notifications out of the pathogen workflows and into the GitHub Action
workflows.

¹ https://github.com/nextstrain/conda-base/blob/5655133be6864ede0acbf40b6b06222f9f2dc031/.github/workflows/ci.yaml#L107-L112
² https://github.com/nextstrain/ncov/blob/fcac1d16410b1b119a9af70afe7b9325a6e25083/workflow/snakemake_rules/export_for_nextstrain.smk#L499-L519

README.md

@@ -95,6 +95,7 @@ See also GitHub's [documentation on starter workflows](https://docs.github.com/e
 
 Executable scripts that are used in our workflows.
 
+- [interpolate-text-template](bin/interpolate-text-template)
 - [write-envdir](bin/write-envdir)
 - [json-to-envvars](bin/json-to-envvars)
 - [yaml-to-envvars](bin/yaml-to-envvars)

bin/interpolate-text-template

@@ -0,0 +1,12 @@
+#!/bin/bash
+# usage: interpolate-text-template <text-template>
+#
+# Replaces ${x} in <text-template> with value of environment variable "x" and
+# outputs the updated text to stdout.
+#
+# Modified from the conda-base repo's CI workflow:
+# https://github.com/nextstrain/conda-base/blob/a2da0f5ef7e95f9db31678c2df00bd312c7b754a/.github/workflows/ci.yaml#L109-L111
+#
+set -eou pipefail
+
+perl -pe 's/\$\{(.+?)\}/$ENV{$1}/ge' < "$1" \