chore(ci): support branches from forks

[pront]

Summary

Follow up to #1398.

Extracted commits from https://github.com/vectordotdev/vrl/pull/1399/files#diff-4f5c849c205cf3224b7c13fdcfc7f684e272f85f2a7a8aae17d5ce4a0d9a7036.

Change Type

• Bug fix
• New feature
• Non-functional (chore, refactoring, docs)
• Performance

Is this a breaking change?

• Yes
• No

How did you test this PR?

#!/bin/bash
set -euo pipefail

# Simulated PR info
VRL_GITHUB_REPO="Zettroke/vrl"
VRL_GITHUB_BRANCH="apply-function-closure-type-info"

# Create a temporary Cargo.toml
cat > Cargo.toml <<EOF
[dependencies]
vrl = { git = "https://github.com/vectordotdev/vrl.git", branch = "main", features = ["cli", "foo", sss] }
tokio = "1.0"
EOF

echo "🔧 Before:"
cat Cargo.toml

# Path to the Cargo.toml file
CARGO_TOML="Cargo.toml"

# Set new git repo and branch
VRL_GITHUB_REPO="some_fork"
VRL_GITHUB_BRANCH="some_branch"
NEW_GIT="https://github.com/${VRL_GITHUB_REPO}.git"
NEW_BRANCH="${VRL_GITHUB_BRANCH}"

# Extract existing features (if any)
EXISTING_FEATURES=$(sed -nE 's/.*vrl = \{[^}]*features = (\[[^]]*\]).*/\1/p' "$CARGO_TOML")

# Compose the new dependency line
if [[ -n "$EXISTING_FEATURES" ]]; then
  NEW_LINE="vrl = { git = \"$NEW_GIT\", branch = \"$NEW_BRANCH\", features = $EXISTING_FEATURES }"
else
  NEW_LINE="vrl = { git = \"$NEW_GIT\", branch = \"$NEW_BRANCH\" }"
fi

# Replace the old vrl line
sed -i.bak -E "s|vrl = \{[^}]*\}|$NEW_LINE|" "$CARGO_TOML"



echo -e "\n✅ After:"
cat Cargo.toml

Does this PR include user facing changes?

• Yes. Please add a changelog fragment based on
  our guidelines.
• No. A maintainer will apply the "no-changelog" label to this PR.

Checklist

• Our CONTRIBUTING.md is a good starting place.
• If this PR introduces changes to LICENSE-3rdparty.csv, please
  run dd-rust-license-tool write and commit the changes. More details here.
• For new VRL functions, please also create a sibling PR in Vector to document the new function.

References

[pront]

Merged so we can use it on open PRs and add it to required checks.