feat: Add a Flox environment for running (unit) tests

Use Flox [1] to define a minimal environment to run ORT's (unit) tests.
Test can be run in isolation from the host system [2] via

    env -i "$(which flox)" activate -- ./gradlew test

This helps to verify that these tests indeed have no dependencies on
external tools (except those present in the Flox environment).

All files in `.flox` were initially auto-generated by `flox init` [3].
The only file that should ever be manually edited is `manifest.toml` via
`flox edit`, but for adding packages using the combination of
`flox search` and `flox install` is actually more convenient.

[1]: https://flox.dev/docs/
[2]: flox/flox#2447
[3]: https://flox.dev/docs/tutorials/creating-environments/#initialize-a-project

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

Author: sschuberth

.flox/.gitattributes

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

.flox/.gitignore

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

.flox/env.json

@@ -0,0 +1,4 @@
+{
+  "name": "ort-flox",
+  "version": 1
+}

.flox/env/manifest.lock

@@ -0,0 +1,18 @@
+# Flox manifest version managed by Flox CLI
+version = 1
+
+[vars]
+FLOX_DISABLE_METRICS = "true"
+
+[options]
+activate.mode = "run"
+cuda-detection = false
+systems = ["x86_64-linux"]
+
+[install]
+jdk21.pkg-path = "jdk21"
+
+# Required to run the Gradle wrapper:
+coreutils.pkg-path = "coreutils"
+findutils.pkg-path = "findutils"
+gnused.pkg-path = "gnused"