fix: pin Terraform 1.15.5 for all Nix platforms (#25799)

The terraform_1_15_5 derivation previously only handled linux/amd64,
falling through to unstablePkgs.terraform on all other platforms. On
macOS this meant a different Terraform version was used, which caused
the version check in make pre-commit to trigger generate.sh,
regenerating all testdata with the host platform's OS/arch
(darwin/arm64) instead of the committed linux/amd64 values.

Three changes:

1. `flake.nix`: add explicit linux_arm64, darwin_arm64, and darwin_amd64
cases with SHA256 hashes from the official HashiCorp release. Unknown
platforms still fall back to unstablePkgs.terraform.

2. `provisioner/terraform/testdata/generate.sh`: guard full regeneration
behind a Linux-only check. The committed testdata encodes linux/amd64
values from the coder_provisioner data source, so regenerating on macOS
would permanently bake in darwin/arm64. The --check path still runs on
all platforms so the version target can detect provider mismatches.
Regeneration via CI or an explicit Linux run is unchanged.

3. `scripts/release/check_commit_metadata.sh`: fix a shfmt (>=3.13)
false positive. The [install.sh] key in an associative array literal was
parsed as floating-point arithmetic (a zsh-only feature). Moving it to a
post-declaration assignment satisfies the stricter parser without
changing runtime behavior.

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Linear: DOCS-279

provisioner/terraform/testdata/generate.sh

@@ -140,6 +140,17 @@ if [[ " $* " == *" --check "* ]]; then
 	fi
 fi
 
+# Committed testdata encodes linux/amd64 values from coder_provisioner.
+# Regenerating elsewhere bakes in the host OS/arch.
+if [[ "$(uname)" != "Linux" ]]; then
+	if ((upgrade)); then
+		echo "ERROR: --upgrade is not supported on $(uname); run on Linux or via CI."
+		exit 1
+	fi
+	echo "Note: skipping testdata regeneration on $(uname); regenerate on Linux or via CI."
+	exit 0
+fi
+
 # Filter flags from positional args to get directory names.
 declare -a dirs=()
 for arg in "$@"; do