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.

<!--

If you have used AI to produce some or all of this PR, please ensure you
have read our [AI Contribution
guidelines](https://coder.com/docs/about/contributing/AI_CONTRIBUTING)
before submitting.

-->



Linear: DOCS-279

scripts/release/check_commit_metadata.sh

@@ -78,7 +78,6 @@ main() {
 		[enterprise]="Enterprise"
 		[examples]="Examples"
 		[helm]="Helm"
-		[install.sh]="Installer"
 		[provisionersdk]="SDK"
 		[provisionerd]="Provisioner"
 		[provisioner]="Provisioner"
@@ -88,6 +87,8 @@ main() {
 		[support]="Support"
 		[tailnet]="Networking"
 	)
+	# shfmt (>=3.13) parses [install.sh] as floating-point arithmetic in array literals.
+	humanized_areas["install.sh"]="Installer"
 
 	# Get hashes for all cherry-picked commits between the selected ref
 	# and main. These are sorted by commit title so that we can group