build(flox): Add composite envionments with external tools

At the example of scanners only for now, show how to create Flox
environments for their tools so that functional tests for scanners can
be run independently of whether they are currently installed via:

    ./flox/run_isolated.sh external-tools ./gradlew -p plugins/scanners \
        funTest

Note that if an included environment changes, the compose environment
needs to be updated [1] by running

    flox include upgrade

[1]: https://flox.dev/docs/concepts/composition/#getting-the-latest-manifests

Signed-off-by: Sebastian Schuberth <[email protected]>

Author: sschuberth

flox/external-tools/.flox/.gitattributes

@@ -0,0 +1 @@
+env/manifest.lock linguist-generated=true linguist-language=JSON

flox/external-tools/.flox/.gitignore

@@ -0,0 +1,5 @@
+run/
+cache/
+lib/
+log/
+!env/

flox/external-tools/.flox/env.json

@@ -0,0 +1,4 @@
+{
+  "name": "external-tools",
+  "version": 1
+}