install.sh: POSIX-compatible 'set -o pipefail' so 'curl | sh' works

The README documents the one-liner as 'curl ... install.sh | sh',
but on Debian/Ubuntu /bin/sh is dash which rejects 'set -o pipefail'
unknown option. The shebang #!/usr/bin/env bash is honored only
when the script is invoked directly — when piped via 'curl | sh'
the running shell IS dash.

Fix: split the strict-mode setup. 'set -eu' is POSIX-portable
(every shell). 'pipefail' is then enabled conditionally only on
shells that recognise it. Every curl/tar/install step in the rest
of the script checks its own exit code, so losing pipefail in dash
costs no behaviour — the installer still fails fast on any error.

install.sh

@@ -19,7 +19,12 @@
 #   0 — installed successfully
 #   1 — error (unsupported arch, download failure, permission denied)
 
-set -euo pipefail
+# POSIX-friendly: -eu is universal, pipefail only on shells that
+# support it (bash, ksh, dash >= 0.5.12). Without pipefail the
+# installer still exits on the first hard error since every curl/
+# tar/install step is checked explicitly.
+set -eu
+(set -o pipefail) 2>/dev/null && set -o pipefail || true
 
 REPO="${SKELETONKEY_REPO:-KaraZajac/SKELETONKEY}"
 VERSION="${SKELETONKEY_VERSION:-latest}"