Add --environ-file default for env (using dotenvx).

If the --environ-file option is used then dotenvx is used to load and
parse the environment file to use a defaults env values.

Add an example suite examples/03-env.yaml that is designed to be used
with --environ-file examples/03-env-file.

Add tests to test/runexamples that tests examples/03-env.yaml with and
without the the examples/03-env-file.

NOTE: It would be preferable to have the option be named `--env-file`
since this is more common. However, when running in nbb with node
earlier than 22, node intercepts the --env-file option. And for some
reason, node refused absoluate paths with --env-file. In node v22 it is
possible to avoid the issue by using a "--" parameter before the
regular nbb script parameters. But node 22 is not in standard GHA runner
configs so for now we will use "--environ-file".

examples/03-env-file

@@ -0,0 +1,4 @@
+BAR=x
+BAR=${BAR:+c}
+BAR="b${BAR:-C}d"
+BAR="a${BAR:-SEE}e"

examples/03-env.yaml

@@ -0,0 +1,64 @@
+name: Example Suite - Environment variables
+
+# requires use of `--environ-file examples/03-env-file` to succeed
+
+env:
+  FOO: p1-${{ env.FOO || "foo" }}-s1
+  BAR: p1-${{ env.BAR || "bar" }}-s1
+
+tests:
+
+  env-depth-inside-FOO:
+    name: multiple levels, FOO inside
+    env:
+      FOO: p2-${{ env.FOO }}-s2
+    steps:
+      - exec: node1
+        env:
+          FOO: p3-${{ env.FOO }}-s3
+        run: |
+          FOO="p4-${FOO:-invaid}-s4"
+          echo "inside FOO: '${FOO}'"
+          [ "p4-p3-p2-p1-foo-s1-s2-s3-s4" = "${FOO}" ]
+
+  env-depth-inside-BAR:
+    name: multiple levels, BAR inside
+    env:
+      BAR: p2-${{ env.BAR }}-s2
+    steps:
+      - exec: node1
+        env:
+          BAR: p3-${{ env.BAR }}-s3
+        run: |
+          BAR="p4-${BAR:-invaid}-s4"
+          echo "inside BAR: '${BAR}'"
+          [ "p4-p3-p2-p1-abcde-s1-s2-s3-s4" = "${BAR}" ]
+
+  env-depth-outside-FOO:
+    name: multiple levels, FOO outside
+    env:
+      FOO: p2-${{ env.FOO }}-s2
+    steps:
+      - name: FOO outside
+        exec: :host
+        env:
+          FOO: p3-${{ env.FOO }}-s3
+        run: |
+          FOO="p4-${FOO:-invalid}-s4"
+          echo "outside FOO: '${FOO}'"
+          [ "p4-p3-p2-p1-foo-s1-s2-s3-s4" = "${FOO}" ]
+
+  env-depth-outside-BAR:
+    name: multiple levels, BAR outside
+    env:
+      FOO: p2-${{ env.FOO }}-s2
+      BAR: p2-${{ env.BAR }}-s2
+    steps:
+      - name: BAR outside
+        exec: :host
+        env:
+          BAR: p3-${{ env.BAR }}-s3
+        run: |
+          BAR="p4-${BAR:-invalid}-s4"
+          echo "outside BAR: '${BAR}'"
+          [ "p4-p3-p2-p1-abcde-s1-s2-s3-s4" = "${BAR}" ]