Fix installer downloads for specific releases

Github download urls for a specific release uses the pattern `../releases/download/[RELEASE]/..`, which differs from the latest release URL which uses `../releases/latest/download/..`.

For convenience, if `BINSTALL_VERSION` is set but doesn't start with `v`, the v is prefixed.

install-from-binstall-release.ps1

@@ -2,10 +2,19 @@ $ErrorActionPreference = "Stop"
 Set-PSDebug -Trace 1
 $tmpdir = $Env:TEMP
 $BINSTALL_VERSION = $Env:BINSTALL_VERSION
-if (-not $BINSTALL_VERSION) {
-    $BINSTALL_VERSION = 'latest'
+if ($BINSTALL_VERSION -and $BINSTALL_VERSION -notlike 'v*') {
+    # prefix version with v
+    $BINSTALL_VERSION = "v$BINSTALL_VERSION"
 }
-$base_url = "https://github.com/cargo-bins/cargo-binstall/releases/$BINSTALL_VERSION/download/cargo-binstall-"
+# Fetch binaries from `[..]/releases/latest/download/[..]` if _no_ version is
+# given, otherwise from `[..]/releases/download/VERSION/[..]`. Note the shifted
+# location of '/download'.
+$base_url = if (-not $BINSTALL_VERSION) {
+    "https://github.com/cargo-bins/cargo-binstall/releases/latest/download/cargo-binstall-"
+} else {
+    "https://github.com/cargo-bins/cargo-binstall/releases/download/$BINSTALL_VERSION/cargo-binstall-"
+}
+
 $proc_arch = [Environment]::GetEnvironmentVariable("PROCESSOR_ARCHITECTURE", [EnvironmentVariableTarget]::Machine)
 if ($proc_arch -eq "AMD64") {
 	$arch = "x86_64"

install-from-binstall-release.sh

@@ -2,11 +2,21 @@
 
 set -euxo pipefail
 
-BINSTALL_VERSION="${BINSTALL_VERSION:-latest}"
+if [[ -n "$BINSTALL_VERSION" && "$BINSTALL_VERSION" != v* ]]; then
+    # prefix version with v
+    BINSTALL_VERSION="v$BINSTALL_VERSION"
+fi
 
 cd "$(mktemp -d)"
 
-base_url="https://github.com/cargo-bins/cargo-binstall/releases/${BINSTALL_VERSION}/download/cargo-binstall-"
+# Fetch binaries from `[..]/releases/latest/download/[..]` if _no_ version is
+# given, otherwise from `[..]/releases/download/VERSION/[..]`. Note the shifted
+# location of '/download'.
+if [[ -z "$BINSTALL_VERSION" ]]; then
+    base_url="https://github.com/cargo-bins/cargo-binstall/releases/latest/download/cargo-binstall-"
+else
+    base_url="https://github.com/cargo-bins/cargo-binstall/releases/download/${BINSTALL_VERSION}/cargo-binstall-"
+fi
 
 os="$(uname -s)"
 if [ "$os" == "Darwin" ]; then