fix(install): Respect TARGETARCH env var for cross-compilation

The install script now checks for TARGETARCH (set by Docker buildx)
or ARCH environment variables before falling back to `uname -m`.

This fixes cross-compilation scenarios where `uname -m` returns the
build host architecture instead of the target architecture, causing
the wrong binary to be downloaded.

Example usage in Dockerfile:
```dockerfile
ARG TARGETARCH
RUN wget -O- .../install.sh | sh
```

Co-Authored-By: Claude <noreply@anthropic.com>
Signed-off-by: Aleksei Sviridkin <f@lex.la>

Author: lexfrei

hack/install.sh

@@ -18,6 +18,11 @@ Usage: $0 [OPTIONS]
 Options:
   -v, --version VERSION   Install specific release (e.g. 1.4.0 or v1.4.0).
   -h, --help              Show this help and exit.
+
+Environment variables:
+  TARGETARCH              Override architecture detection (e.g. amd64, arm64).
+                          Useful for Docker buildx cross-compilation.
+  ARCH                    Alternative to TARGETARCH for architecture override.
 EOF
 }
 
@@ -89,7 +94,16 @@ else
 fi
 
 OS=$(uname -s | tr '[:upper:]' '[:lower:]')
-ARCH=$(uname -m)
+
+# Detect architecture: respect TARGETARCH (Docker buildx) or ARCH env vars, fallback to uname -m
+if [ -n "$TARGETARCH" ]; then
+  ARCH="$TARGETARCH"
+elif [ -n "$ARCH" ]; then
+  # ARCH already set by environment
+  :
+else
+  ARCH=$(uname -m)
+fi
 
 case "$ARCH" in
   x86_64|amd64) ARCH="amd64" ;;