Enforce shell linting/formatting and modernize scripts

[runesoerensen]

Add shell script linting (ShellCheck) and formatting (shfmt) enforcement, and modernize all shell scripts to follow consistent conventions.

Linting & formatting infrastructure

• Add .shellcheckrc with enable=all for maximum strictness
• Add .editorconfig configuring shfmt (tabs, binary_next_line, switch_case_indent, shell_variant=bash)
• Add Makefile targets: lint (meta), lint-scripts (ShellCheck), check-format / format (shfmt)
• Add lint job to GitHub Actions CI (ShellCheck + shfmt)
• Exclude vendored test/shunit2.sh from both linting and formatting
• Add .editorconfig and .shellcheckrc to buildpack.toml publish ignore list

ShellCheck fixes

• Quote all variable expansions ($var -> "${var}")
• Replace [ ] with [[ ]] for bash-specific tests
• Replace -a/-o compound tests with &&/||
• Replace [ ! -z with [[ -n
• Fix unquoted array expansions and glob patterns
• Add targeted # shellcheck disable= directives for intentional patterns (e.g. word-splitting of $pkgs, dynamically sourced files)

shfmt formatting

• Normalize all indentation to tabs (consistent with heredoc <<- requirement)
• Apply switch_case_indent, binary_next_line formatting rules
• Consistent spacing around redirections (>, >>, <, &>)

Shell script modernization

• Shebangs: Standardize to #!/usr/bin/env bash across all scripts (including converting sbin/add-version from #!/bin/sh and adding a shebang to test/utils.sh)
• BUILDPACK_DIR: Replace $0-based path resolution with BASH_SOURCE[0] pattern (BUILDPACK_DIR=$(cd "$(dirname "$(dirname "${BASH_SOURCE[0]}")")" && pwd)) — more reliable when sourced, and lets ShellCheck follow source paths without # shellcheck source= hints
• set -euo pipefail: Enable strict error handling everywhere (-u for unset variable detection, -o pipefail for pipeline failures). Add :- defaults to all optional/environment variables (${GOVERSION:-}, ${GO_INSTALL_PACKAGE_SPEC:-}, etc.)
• Explicit source dependencies: Move output.sh sourcing out of common.sh into each caller, making dependency chains visible
• Remove unnecessary operations: Drop mkdir -p "$1" "$2" (platform guarantees dirs exist), cd "$1/" && pwd canonicalization (platform passes absolute paths), and a broken no-op pushd/popd in test/run.sh
• Variable naming: Rename all buildpack API variables to uppercase convention:
  • build -> BUILD_DIR
  • env_dir -> ENV_DIR
  • cache_root -> CACHE_DIR
  • cache -> GO_CACHE_DIR (the namespaced .heroku/go subdirectory)
  • buildpack/testpack -> BUILDPACK_DIR

GUS-W-21598655