Description of problem:
THe command will exit when the command `operator-sdk pkgman-to-bundle` run with flag "--build-cmd".

Version-Release number of selected component (if applicable):
operator-sdk version: "v1.8.0-ocp", commit: "280f5d34954df1cb8176dafc35551feaa8ff9e1b", kubernetes version: "v1.20.2", go version: "go1.15.5", GOOS: "linux", GOARCH: "amd64"

How reproducible:
always

Steps to Reproduce:
use the etcd packagemanifest: https://github.com/operator-framework/community-operators/tree/master/community-operators/etcd  

1.# operator-sdk pkgman-to-bundle etcd/ --output-dir ./etcdbundle --build-cmd "docker"  --image-tag-base quay.io/jfan/etcdddd
INFO[0000] Packagemanifests will be migrated to bundles in ./etcdbundle directory 
INFO[0000] Creating etcdbundle/bundle-0.6.1/bundle.Dockerfile 
INFO[0000] Creating etcdbundle/bundle-0.6.1/metadata/annotations.yaml 
INFO[0000] Bundle metadata generated suceessfully       
INFO[0000] Using the specified command to build image quay.io/jfan/etcdddd:0.6.1 
Error: exit status 1

2. operator-sdk pkgman-to-bundle etcd/ --output-dir ./etcdbundle --build-cmd "podman"  --image-tag-base quay.io/jfan/etcdddd
INFO[0000] Packagemanifests will be migrated to bundles in ./etcdbundle directory 
INFO[0000] Creating etcdbundle/bundle-0.6.1/bundle.Dockerfile 
INFO[0000] Creating etcdbundle/bundle-0.6.1/metadata/annotations.yaml 
INFO[0000] Bundle metadata generated suceessfully       
INFO[0000] Using the specified command to build image quay.io/jfan/etcdddd:0.6.1 
Error: exit status 125


3. the PATH
# echo $PATH
/usr/share/Modules/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/usr/local/go/bin:/root/bin:/usr/bin/docker

Actual results:
The command will exit status 1 with `--build-cmd "docker" `
and exit status 125 with `--build-cmd "podman"`

Expected results:
The command could success with the flag --build-cmd

Additional info:
The command can build the image success without flag "--build-cmd"
# operator-sdk pkgman-to-bundle etcd/ --output-dir ./etcdbundle   --image-tag-base quay.io/jfan/etcdddd
INFO[0000] Packagemanifests will be migrated to bundles in ./etcdbundle directory 
INFO[0000] Creating etcdbundle/bundle-0.6.1/bundle.Dockerfile 
INFO[0000] Creating etcdbundle/bundle-0.6.1/metadata/annotations.yaml 
INFO[0000] Bundle metadata generated suceessfully       
INFO[0000] Successfully built image quay.io/jfan/etcdddd:0.6.1 
INFO[0000] Supported channels cannot be identified from CSV etcdoperator.v0.9.0, using default channel 'preview' 
INFO[0000] Creating etcdbundle/bundle-0.9.0/metadata/annotations.yaml 
INFO[0000] Creating etcdbundle/bundle-0.9.0/bundle.Dockerfile 
INFO[0000] Bundle metadata generated suceessfully       
INFO[0001] Successfully built image quay.io/jfan/etcdddd:0.9.0 
INFO[0001] Supported channels cannot be identified from CSV etcdoperator.v0.9.2, using default channel 'preview' 
INFO[0001] Creating etcdbundle/bundle-0.9.2/bundle.Dockerfile 
INFO[0001] Creating etcdbundle/bundle-0.9.2/metadata/annotations.yaml 
INFO[0001] Bundle metadata generated suceessfully       
INFO[0002] Successfully built image quay.io/jfan/etcdddd:0.9.2 
INFO[0002] Supported channels cannot be identified from CSV etcdoperator.v0.9.2-clusterwide, using default channel 'preview' 
INFO[0002] Creating etcdbundle/bundle-0.9.2-clusterwide/bundle.Dockerfile 
INFO[0002] Creating etcdbundle/bundle-0.9.2-clusterwide/metadata/annotations.yaml 
INFO[0002] Bundle metadata generated suceessfully       
INFO[0003] Successfully built image quay.io/jfan/etcdddd:0.9.2-clusterwide 
INFO[0003] Creating etcdbundle/bundle-0.9.4/bundle.Dockerfile 
INFO[0003] Creating etcdbundle/bundle-0.9.4/metadata/annotations.yaml 
INFO[0003] Bundle metadata generated suceessfully       
INFO[0004] Successfully built image quay.io/jfan/etcdddd:0.9.4 
INFO[0004] Creating etcdbundle/bundle-0.9.4-clusterwide/bundle.Dockerfile 
INFO[0004] Creating etcdbundle/bundle-0.9.4-clusterwide/metadata/annotations.yaml 
INFO[0004] Bundle metadata generated suceessfully       
INFO[0004] Successfully built image quay.io/jfan/etcdddd:0.9.4-clusterwide